If you are using the latest version of iMovie (2.2.3) here is the instructions from the support documents:
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Manage media copied from another device
You can edit media used in projects that have been shared between devices in the iMovie Media section of the media library. There you can download the media to your Photos library, or delete the media from the current device.
Note: Media from projects created in previous versions of iMovie can also be managed this way.
- Tap Projects to open the Project Viewer, select the project with the media you want to manage, and tap Edit.
- Tap the Add Media button
, then tap Videos. - Select iMovie Media.Any video clips that are not already saved in your Photos library appear.
- Tap Edit.Buttons appear at the right side of each clip.
- Tap the gray Download button
to save the clip to your Photos library. - Tap the red Delete button
to remove the clip from your local device.Note: If the clip is currently used in a project, you must remove it from the Timeline before deleting it. For more, see Remove a video clip or photo from a project.
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I believe the process is very similar for previous versions of iMovie, though the names/icons may be different. Hope that helps