Windows 10 is available as a free upgrade from Windows 7 or Windows 8.1. Here's what you need to know before you upgrade, including details about activation and product keys. [Updated December 2, 2015] By Ed Bott for The Ed Bott Report | December 3, 2015 -- 00:41 GMT (16:41 PST) [Latest update: December 2, 2015. This article has gone through multiple updates since its first publication in late 2014. The current revision is based on the most recent public release of Windows 10, version 1511, released in November 2015.] This article covers the most common scenarios you're likely to encounter when installing and activating Windows 10. What's the most recent Windows 10 version? Microsoft released Windows 10 to the general public on July 29, 2015, following a nine-month-long preview. As part of its new "Windows as a Service" development schedule, the company released a major upgrade, version 1511, on November 12, 2015. The build number of the July 2015 release is 10240. The build number of the November 2015 release is 10586. To check which version of Windows 10 is installed on a device, open Settings, click System, then click About. As an alternative, you can click in the search box, type winver, and view results in a compact About Windows dialog box. You can also view the build number (but not version information) using the System Information app, msinfo32. Look near the top of the System Summary page for the build number. (If you're running a preview build as part of the Windows Insider program, you'll see an Insider Preview label after the Windows 10 edition, accompanies by a build number that's higher than the latest release version.) Is it possible to upgrade Windows 7 or 8.1 to Windows 10 using Windows Update? Since the initial release date, Microsoft has delivered Windows 10 online via Windows Update to people who placed an online reservation. On systems that are eligible for the free Windows 10 upgrade where automatic updating is enabled, the Windows 10 installer files download automatically in the background. This is what the installer looks like in Windows Update on a system running Windows 7. What's the best way to upgrade without waiting for Windows Update? The latest Windows 10 version is available from the Get Windows 10 page in two formats. Use the Upgrade Now button to immediately begin upgrading the current PC running Windows 7 or 8.1 If you plan to upgrade a different system (or multiple systems), scroll down the Get Windows 10 page and download the media creation tool. Running this tool allows you to create a bootable USB flash drive or download the Setup files in ISO format. After running the Media Creation Tool, choose Create installation media for another PC. By default, the pre-selected options choose the version that matches the current PC. Clear the3 check box shown here if you want to choose a different version, such as downloading the x86 version on a 64-bit PC. The download files contain both Home and Pro editions. The ISO file for an x86 (32 GB) edition is approximately 3 GB in size. The x64 (64-bit) edition is delivered as an ISO file approximately 4 GB in size. What's the difference between Windows 10 and Windows 10 N? The N editions of Windows lack media playback capabilities. Microsoft is required to offer these editions as part of a settlement in a European antitrust trial. You don't want it. Nobody does. Will the Windows 10 upgrade happen automatically? Beginning in early 2016, Microsoft will offer the Windows 10 upgrade as a Recommended update. When that happens, anyone who has configured Windows Update to install Recommended updates automatically will see the Setup program run after the update finishes downloading. The Setup program requires you to click to run it. It also requires acceptance of the Windows license agreement. Unless the owner of the device to be upgraded provides affirmative consent to both of these prompts, the upgrade will not continue. Will my PC run Windows 10? Most PCs that can run Windows 8.1 will run Windows 10. The Windows setup program runs a compatibility check at the beginning of the process and will halt the upgrade and describe the problem if it finds a blocking issue. Do I need a product key? If you upgrade over a properly activated copy of Windows 7 or Windows 8.1, you do not need a product key. After the upgrade is complete, your new copy of Windows 10 will be activated with what Microsoft calls a "digital entitlement." If you are using bootable installation media to perform a clean install on a PC that was previously upgraded and given a digital entitlement, you do not need to enter a product key. The system will be activated after the installation is complete. If you are using bootable installation media to perform a clean install on a PC that has never been upgraded to Windows 10 and activated, you will need to enter a product key. Beginning with build 10586, you can enter a product key from a matching edition of Windows 7, Windows 8, or Windows 8.1. To check the activation status of a Windows 10 PC, open Settings, click Update & Security, then click Activation. Should I upgrade or do a clean install? That's up to you. Windows 10 supports upgrades over Windows 7 Service Pack 1 or later as well as clean installs. To do a clean install, you will need bootable media. Can I set up Windows 10 as part of a dual- or multi-boot installation? Yes, Windows 10 will add itself to the boot menu if you install it on a system that already has a previous version of Windows installed. An important note: When you use a conventional installation, you must install Windows 10 on a separate partition or physical drive; if you choose the same partition that contains an existing Windows version, you will lose access to that Windows installation. (Your files will be moved to the Windows.old folder.) Is it possible to uninstall Windows 10? Yes. For 30 days after the upgrade, you can roll back to your previous Windows version if you experience problems. The option to roll back is in Settings, on the Recovery tab under Update & Security. Can I install Windows 10 in a virtual machine? Yes, as long as the virtualization software is compatible. I have had no problems running test installations in several Hyper-V virtual machines on Windows 8.l and Windows 10. How do I install the November Update if Windows 10 is running on a VHD using native boot? A quick explainer for those wondering what that question even means. Native boot is a feature that was introduced with Windows 7 and is documented in this Knowledge Base article. With native boot, you can install a business edition of Windows 7 or later in a Virtual Hard Disk (VHD) file. The setup instructions in this article appear to be valid with the Windows 10 Technical Preview, in my testing. The advantage of installing on a VHD is that you don't have to mess with disk partitions. Windows mounts the VHD file when it starts and treats it as though it were a physical disk. Your main OS remains on the disk itself, allowing you to easily set up a dual-boot system to see how Windows 10 will run on your hardware without creating a mess. Although it's not well documented, this type of installation has a significant limitation. As this TechNet article notes, "If you boot from a VHD, you cannot upgrade the Windows version in the VHD to a newer version." The only alternative is cumbersome. The short version is this: Create a new Hyper-V virtual machine using the VHD file (make a copy, if possible). Boot the VM from the Windows 10 ISO (attached as a virtual DVD). Run startup repair on the virtual machine. Start the VM and upgrade to the new build. After the upgrade is complete, shut down the VM, copy the updated VHD file to the location specified in the boot menu. Restart the PC, choosing the VHD option from the boot menu. Those instructions are obviously incomplete and assume you are familiar with some very advanced Windows tools and techniques: Hyper-V, the Windows multi-boot process, and mounting VHD files. If you're unsure about any of those topics, I suggest that you steer clear of native boot for now, or use it only in test environments where you're not concerned about the possibility of data loss. If you're willing to accept that extra bit of hassle, follow the steps in this post: How to use a VHD to dual-boot Windows 8 on a Windows 7 PC. How do I send feedback to Microsoft? You'll see occasional pop-up prompts that ask questions about your experience with Windows 10. You can click those tiles to send comments, or use the Windows Feedback button on the new Start menu.