How to downgrade from iOS 15 to 14.6

First you need to delete the beta 15...

How to uninstall iOS beta software - Apple Support

Then you can update...

Update your iPhone, iPad, or iPod touch - Apple Support

from my knowledge, if your phone is not working at all, you'll need to restore it's data with the help of a Mac. just hook it up with a plug and the option 'Restore Phone' will show up in Finder. (Note: Restoring a Phone will delete all of it's data.)

it was advised to backup your device to a computer before updating in case this would have occurred. you'll need to reinstall iOS 14 with your Mac.

Any computer with iTunes installed will do. Follow these instructions to restore the phone...

If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

If you didn't do as you were instructed before you went to the Beta software, yes, you'll lose your data.

you'll need to download iTunes on the Windows Store and log into your Apple account if you want to restore your iPhone.

Check this link...

Update to the latest version of iTunes - Apple Support

Or make an appointment at the nearest Apple Store for assistance...

Apple Store - Find a Store - Apple