While standing on your head ...
..., open your Display settings and look for an option to rotate your display. In my system, using nVidia graphics cards, the option is titled NVRotate.
I understand that, on some systems, the screen can be rotated by using certain keys on the keyboard, but that doesn't work in my system.
Edit: Ah, it's Ctrl/Alt plus arrow keys that supposedly will do the trick on some systems.