rotate a video

This is not yet supported in Photos 5 on macOS 10.15 Catalina, but will be supported in Photos 6 on macOS 10.16 Big Sur.

For the time being, export the video from Photos, then open the exported video in QuickTime Player and use the command from the Editor menu:

• Edit > Rotate Left ⇧⌘L
• Edit > Rotate Right ⇧⌘R

Save the video and import it again to Photos.

Export the original version to the Desktop and open with Quicktime Player. Then use the Edit menu and select Rotate Left or Rotate Right as the case may be. Then use the File ➙ Export menu to save the file in the new orientation.