Need to go back iOS 15 from iOS 16

Sorry, but you will need a computer to downgrade your iPhone back to iOS 15. See if a friend or relative can assist you with this. Alternately, visit your local Apple Store or Apple Authorized Service Provider where they can help you downgrade your phone.

Ref: Restore Shipping Software

You need a computer as it involves DFUing the phone. Borrow one.

The only question regarding Betas that we can answer is how to uninstall them.

How to uninstall iOS or iPadOS beta software - Apple Support

Unenroll Your Devices

If your last backup was from iOS 16 then you cannot restore it to iOS15 so take care.