Right Click on image. Select Transform> Rotate Left or Right.
Well actually, it doesn't say it all
Do you mean rotate a few degrees to get the horizon level, or 90 degrees because the camera or software didn't properly recognize the photo was either landscape or portrait, or 180 degrees; or change from landscape to portrait without rotating 90 degrees; or what?
Ctrl (Cmd - Mac) And the left and right bracket keys will rotate images 90° clockwise or counterclockwise.
use command +[ to rotate left
command + ] to rorate right