iMovie Library.imovielibrary file to open it in iMovie.
Open any project like you would normally do for editing.
In the sidebar under the LIBRARIES heading, click your library name once (the default library name is iMovie Library).
Wait a few seconds and then click it a second time. You will now be able to edit the name. Type the new, desired name (for example, External iMovie Library) and press your Enter key.
You're all done!