This is something that I get asked reasonably often and thought I’d make a quick video to show just how easy it is.
The process is simple; open the video in question in QuickTime, go up to the Edit menu, go down to the Rotate Left or Rotate Right and select it. You’ll see there are also options to flip the video horizontally or vertically as well.
Once done, click to close the window, and save the video. All done!