Step 1. Open iMovie & Click on import.
Step 2. Select files/videos which needs to be rotated & click on import button.
Step 3. Click on + (Create) button from the toolbar & click on Movie as indicated in following snap-shot.
Step 4. Select “No theme” as we just wish to update the orientation of existing video.
Step 5. In the Movie, Select your video & drag-drop to bottom time-line (as indicated by following snap-shot)
Step 6. From the bottom time-line, select entire visible frame & click on Adjust button from the top-tool-bar
Step 7. See following picture for applying rotation to your video.
Step 8. Click on Share & choose file to save Video.
Step 9. Update the details of video as required & save it.