Slow Down, Speed Up and Reverse Video
From Help Wiki
The 'Clip' Tab
Go to the Clip menu and look at your first 3 options: Slow Motion, Fast Forward, and Instant Replay. These will be able to slow down your video by up to 50% or increase the speed by up to 20x. You'll need to highlight the clip you want to edit in the project window. Editing video speed from Events will not work. Keep in mind, as well as changing the video speed this tool will also affect the audio speed of a clip. There are options in iMovie for audio adjustment as well as recording new audio over a sped up or slowed down clip.
