To prevent the Mac App Store from attempting to overwrite your working version with an incompatible newer update, go to System Preferences > Software Update (or App Store preferences) and uncheck "Automatically keep my Mac up to date."
Double-click the iMovie_10.2.5.dmg file. A new Finder window will open showing: imovie 1025 dmg install
Because Apple no longer hosts older versions on the App Store, you must source the DMG file carefully. Only download DMG files from reputable sources or your own Apple ID purchase history. Pirated DMG files are a major vector for malware. To prevent the Mac App Store from attempting
The first launch will ask to:
A: Only on macOS Mojave with an official eGPU enclosure. Performance gains are minimal. Pirated DMG files are a major vector for malware
Remember: Always backup your data before installing older software. And if you cannot find a legitimate DMG, consider using iMovie alternatives like DaVinci Resolve (which still supports older Macs) or upgrading your hardware to run the modern iMovie.