I’ve downloaded the file 12.5 but I’m facing problem with installing

If you still can't install the latest version of iOS, try downloading the update again:

1. Go to Settings > General > [Device name] Storage.
2. Find the iOS update in the list of apps.
3. Tap the iOS update, then tap Delete Update.
4. Go to Settings > General > Software Update and download the latest iOS update.

If you don’t see the update in the list of apps or if the issue happens again, you need to update your device with iTunes.

Try updating iOS after each step.

• Try Forget the WiFi network and add again
• Try another WiFi network 
• Toggle sets automatically off and then on
• Reset your network settings
• Force restart your device
• Remove any beta profiles, if applicable
• If everything fails: - Use iTunes or Finder (for MacOS Catalina+) to update your device instead of over-the-air

You can’t install 12.5, which is an out-of-date version. You can only install 12.5.5.