Without turning the monitor upside down? I don’t actually want to do it, but a co-worker did that a while back accidentally, but didn’t recall what combination of keys did it. (I don’t know what operating system or anything else his computre was on.)
There’s four different techniques enumerated here. It’s going to matter what OS you’re using. On a Windows box, you can try holding down Ctrl-Alt-Down arrow together and see if that does the trick.
CTRL ALT and the arrow keys
If it is enabled on your graphics driver, the magic key combination is CTRL-ALT-(arrow key), and which arrow key you choose determines which way the screen ends up rotating.
You may need to enable graphics rotation in your graphics properties in the control panel. It’s somewhere under advanced settings on most graphics drivers.
ETA: Or. what they said. (Ninja’d!)
Ctrl+Alt+An arrow key will sometimes reorient your monitor with the direction you chose being considered the ‘top’ of the screen. I think it’s actually related to the graphics card rather than the operating system but I’m not sure.