To update a Final Cut Pro X Library you simply open it with the latest version of Final Cut Pro X. But we always recommend backing up critical libraries before updating them. Also you should archive the old version of Final Cut Pro X so you can open your old back up libraries in case of a problem with the latest version of the app. You can right click on the app and choose “Compress”. This will place the app in a zip file, and you can store it where ever you like. Apple does not offer the option to download older versions of the app.