Hi elainep,
Fear not the solution is at hand!
Top left of the screen is a menu bar that can be expanded/collapsed.
When collapsed it's quite small and just says ‘Menu’. If it’s collapsed just click on it to expand it and the button you want is second in from the right. (Be careful not to hit the coloured X next to it as that will close the App!)
Also if you want you can open AR’s ‘Keyboard Shortcuts’ panel by going to; Menu Bar > Edit > Set Keyboard Shortcuts… and go to the last section ‘Window Commands’ where you can set your own key shortcut to toggle in and out of full screen mode.