I'd put off upgrading iMovie on the MacBook Air as version 10.1.3 kept crashing. I reverted to version 10.1.2 from a backup. So about an hour ago I zipped the existing iMovie version & downloaded the update. Next I unzipped the compressed file to the Utilities folder and checked that it worked, which it did. Closing that version, I headed for the new one in the Applications folder. Big surprise! No crashing. So I trashed the older version plus the zipped one, saving 4.6 GB of space, yes, the updated version runs fine now.