Hello! I don't post here much but I am actually doing this very thing now to get rid of the crappy Win 10 1803 update and roll back to 1709.
Create a recovery disk on another PC running Windows 10, any version it doesn't matter, you'll be overwriting the files with the files you download from the above site. Take note of the instructions it mentions:
1. Connect your USB drive to your Surface (use a USB 3.0 drive if you can). (8GB Drive Worked for me)
2. In the search box on the taskbar, type
recovery, and then select
Create a recovery drive. You might be asked to enter an admin password or confirm your choice.
3. In the
User Account Control dialog box, select
Yes.
4. Make sure
Back up system files to the recovery drive isn’t selected, and then select
Next.
5. Select your USB drive, and then select
Next > Create. A number of files need to be copied to the recovery drive, so this might take a while.
6. When it’s done, select
Finish.
7. Go back to the recovery image .zip file that you downloaded and open it.
8. Drag the files from the recovery image folder to the USB recovery drive you created. Then choose to replace the files in the destination.
9. For info about how to use your new recovery drive, see the “How to reset your Surface to factory conditions” section of
Using a Surface USB recovery drive.
Once you have your recovery drive ready to roll this will guide you through booting from it and restoring the factory image: