Save changes to a project
iMovie automatically saves all the changes you make as you work on a project, which means you never have to save changes manually. Also, you can undo all of your changes up to the last time you quit and reopened iMovie.
Changes you make in a project don’t affect the source media (photos, and video from your Events) because iMovie doesn’t alter the originals. As a result, you can restore a video clip or photo to its original state at any time.