Hi gazzab,
Welcome to PC Mechanic !!
The cure can vary from video card to video card. For mine, you right click on the desktop, then click properties. From there, click on Settings, then Advanced. The window that now pops up for me has multiple tabs, one of which is Rotation. There I can set it to one of four choices: right, left, upside down, and normal.
HTH
TwoRails