Sometimes you make a mistake and click the record button on your mobile device and while shooting that video you turn the camera to a different orientation. This will show you how to rotate it 90 degrees.
- Open iMove and import your video
- Drag your movie into the timeline
- Cut out the parts of the video you don't need.
- Select the movie in your timeline
- Choose the "Cropping" tool in the proprieties window
- Choose "Crop to Fill"
- Choose the "Rotate tool" in the direction you need
- You'll want to "Share video" to make it its own MP4.