I m unable to update my iphone 11 to 13.5.1 after repeated attempts.

VPN or proxy connections might prevent your device from contacting the update servers.

• Get help with over-the-air iOS updates - Apple Support
• Remove and download the iOS update again

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 set 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

• Get help with iOS update and restore errors - Apple Support
• If you see an error when you update or restore your iPhone, iPad