Edit > Rotate and Edit > Flip allows you to rotate your sprite at 90 degrees increments and to mirror it. If you’re asking how to rotate it 45 degrees, it’s not possible to do automatically, but you can try doing it with the transform tool (Ctrl+T), while holding shift to snap it to increments of 15 degrees. Also set the algorithm to Rotsprite instead of fast rotation for better results. You’ll have to do some cleanup work afterwards and touch it up a bit, because no matter how good the algorithm, pixels aren’t meant to be rotated at 45 degrees.