If you mean on the Desktop:
It is designed for mouse and keyboard. So you need to move the mouse at the top, then a menu bar will show with tab, and a button to close the web browser, minimize, and restore the web browser to window mode.
But assuming you fall in the situation you are in, where you started the web browser in desktop, put it full screen, then you have no more keyboard and mouse, then all you need to do is right-click anywhere on the page with your finger. So tap and hold for a sec, a square indicator will show under your finger, when it does, let go, and it will right-click. When you do, the menu bar will show, from where you can hit the restore button (found between the minimize and close button of IE), and you'll be out of full screen.
If you mean on Windows 8 App (aka: Modern UI App) of IE:
Simply swipe down from the center, top of the screen, from the black border of the screen, right under the camera of your Surface Pro 2, and just slide it continuously down, all the way at the bottom. It will close, and go back to the start screen.