If you make a mistake or don’t like a change you’ve made to a project, including photos you’ve added, or to an Event (source video), you can undo it. You can progressively undo all of the actions you’ve performed in a project up to the last time you quit iMovie and reopened it.
To undo your last action:
Choose Edit > Undo [action], or press Command-Z.
The Edit menu appears in a light gray bar across the top of your computer screen.