Hi, rizzutaw,
If you have or ever had iMovie 10 on your Mac you can update to the highest version
of iMovie 10 that your operating system will run. To do that, drag your iMovie 10 app
from the Applications folder into the trash, but do not empty the trash. (You can
skip that step if you had iMovie 10 on your Mac before but perhaps deleted it.)
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 10.
Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the
trash (if you put it there) and return it to the Applications folder.
-- Rich