I joined the public beta program. My device is running iOS 9.1 beta. However, I want the legit iOS 9.0 now. No bugs, no lame performance...
What can I do? Because I go to the update settings and there's nothing there.
Yes, doing what you did kinda presumes you actually know what you're doing.
Restore the phone back to the release 9.0. You will not be able to bring a backup from 9.1 back to 9.0, hope you saved a 9.0 or earlier backup before you installed 9.1.