How to roll back from beta ios 15 to 14

Remove the public beta by deleting the beta profile

Go to Settings > General, and tap Profiles & Device Management. Tap the iOS Beta Software Profile. Tap Remove Profile, then restart your device.

How to uninstall iOS beta software - Apple Support

This is not the right forum to discuss beta

When you experience an issue or something that does not work as expected in beta, send your feedback directly to Apple with Feedback Assistant.

These are the instructions:--> How to uninstall iOS beta software - Apple Support

For any further question, you must contact Apple Beta Software Program, using this link Feedback Assistant 

Regards

Giulio