Your best bet is to update to Mojave if you can do that. Then you can get current version of iMovie.
Otherwise you can update to the highest iMovie 10 version that High Sierra can run, that I think is iMovie 10.1.8 or 10.1.9.
To do that, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. Then sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see
your purchases displayed, including iMovie. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash and return it to the Applications folder.
-- Rich