Speeding up or slowing down a clip is a simple process in iMovie—once you know how to do it.
Start by clicking on or mousing over a clip in your timeline, to reveal the little blue icon with a white gear.
Select “Clip Adjustments” from the dropdown menu.
This will open a gray window with a lot of options, which are worth exploring to discover new ways of fine-tuning your movie and achieving complex effects.
You may see this in the middle of the window:
