Q: I want to downgrade my iPhone to ios9

iOS downgrade is not supported.

Then you need to use the information provided you on the public beta website. It explains how to downgrade your device from beta to the latest public release iOS, 9.3.3.

Funny that they don't read first before they install beta.

Guess they got it off somewhere.

You must not be following the instructions completely, because that error is an incorrect ipsw error. Are you trying to use a downloaded ipsw? You should be letting iTunes download the iOS, after making sure you have removed the beta profile.  About iOS beta software - Apple Support

Read here: If you see Error 3194, Error 17, or 'This device isn't eligible for the requested build' in iTunes - Apple Support