how to get back ios 15

Remove the public beta by deleting the beta profile

If you used a computer to install an iOS or iPadOS beta, you need to restore iOS or iPadOS to remove the beta version.

The easiest way to remove the public beta is to delete the beta profile, then wait for the next software update. Here's what to do: 

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

When the next public version of iOS or iPadOS is available, install it to move back to a non-beta version of iOS or iPadOS. If an update is already available, you can install it by going to Settings > General > Software Update, but the iOS or iPadOS version must be later than the version you have.

If you don't want to wait for a software update to install a non-beta version of iOS or iPadOS, follow the steps to restore iOS or iPadOS.

See this link for further reference

How to uninstall iOS or iPadOS beta software - Apple Support

See this support article:--> How to uninstall iOS or iPadOS beta software - Apple Support

Regards

Giulio