Welcome to the forum.
One way would be to edit the sketch or sketch plane that was used to create that body or feature. That's what I would probably do, although there may be other ways, depending on the circumstances. Is this a seperate body of a multi-body part that you're wanting to rotate, a feature, or what?
Ibe Godwin A,
If this is a separate body or surface, you could use the Move/Copy command ("Insert"/"Feature"/"Move/Copy") to rotate the body or surface. If the Translate box is open, click on the downard pointing chevrons to the right of the Rotate box to open it up.
Are you trying to rotate a selected part in an assembly, as alluded to in your title? If so, would need to make sure it's not held in place by mates. After that, click and hold on the part with the right mouse button and then move the mouse.