Q: I got an error 53 upgrading to ios 9.

You cannot downgrade. Apple does not support it. Even deleting the iOS 9 update will not change that. Error 53 is a hardware error.

The tech at Apple is incorrect. Use your warranty and get it serviced.

You might want to try this link and look at the explanation for error 53.  Get help with iOS update and restore errors - Apple Support

Check for hardware issues

Errors: 1, 3, 11, 12, 13, 14, 16, 20, 21, 23, 26, 27, 28, 29, 34, 35, 36, 37, 40, 53, 56, 1002, 1004, 1011, 1012, 1014, 1667, or 1669.

Your device or computer might have a hardware issue that stops the update or restore from completing.

1. Check that your security software and settings aren't blocking your device from communicating with the Apple update server.
2. Then try to restore your iOS device two more times while connected with a cable, computer, and network you know are good.
3. Make sure that your security software and settings are allowing communication between your device and update servers.
4. If you still see the error message when you update or restore, contact Apple support.

If you still see the error message, check for outdated or modified software.

My emphasis added to the number 53.