Since you are running Mojave you should be able to upgrade to Catalina. Click on Apple Log>About this Mac>Software Update and see if it is available. However, iMovie is not part of the operating system so you upgrading to Catalina will not get you iMovie 10.
If you have or ever have had iMovie 10 on your Mac you can get the highest version that your operating system will run by using this procedure: To update/redownload iMovie 10, 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 computer before, but 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 moved it there) and return it to the Applications folder.
-- Rich