CTRL + ALT + Down Arrow changes to Landscape (Flipped) mode. CTRL + ALT + Left Arrow changes to Portrait mode. CTRL + ALT + Right Arrow changes to Portrait (Flipped) mode.
Open Settings. Click on System. Click on Display. Under the "Rearrange your displays" section, select the screen that you want to rotate (if applicable). Under the "Scale and layout" section, use the Display orientation drop-down menu to select one of the orientation: Landscape. Portrait.