Hi, gaylefromny,
Try putting your present iMovie app, the one you want to update, from the Applications folder into the trash, but don't empty the trash. Now go to the Purchases section of the online app store and see if it has an update or download option for iMovie. iMovie 10.1.6 is the latest version that El Capitan can run. Hopefully it will update to that version.
If the update doesn't work or is unavailable, remove iMovie from the trash and return it to the applications folder.
If you are upgrading from iMovie 9 you will need to have version 9.0.9.
-- Rich