According to one thread on discussions.apple.com, you can use the Apple finder/desktop:
- Right click the file
- Select "Get Info"
- Select "Artwork"
- Select "Add..."
Alternatively, support.apple.com has a legacy article about updating it via iTunes:
- In iTunes, play the movie in a separate window (not in full screen or the main window)
- Drag the playhead to the desired frame
- Right-click (or Control-click) the movie window, and from the shortcut menu, choose Set Poster Frame