If you're playing back with VLC:
Go to File-->Preferences-->Click the "All" button at the bottom. Then, in the side bar: Drop down "Video"-->Drop down "Filters"-->Transformation-->Choose the 90 degree rotation increment you want from the drop down that shows up. Click save and close the preferences window.
Then, open up your file. You then need to go to Window (In the top menu bar)-->Extended Controls. In that popup, check the box that says "transformation." That will start the video with the rotation you specified earlier.
Mind you, this will only rotate the playback, not the actual file. To modify the video file you'll need to use iMovie or some other video editor depending on the video format.