Try these steps to export the video from Photos, open it in QuickTime, and reimport a new copy into Photos:
- Hold down the Option key and drag the video from Photos to the Desktop, or choose File > Export > Export Unmodified Original.
- Find the video you just exported on the Desktop and double-click to open it in QuickTime Player.
- Choose File > Export, select a video quality setting, and export a new copy of the video to your Desktop.
After you've exported the video from QuickTime, you can bring it back into your Photos library by choosing File > Import and selecting the new copy of the video from your Desktop. You can also drag the video from your Desktop into the Photos window.