How do I get rid of the iOS 15 beta?

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.

I am unable to update iOS 15 in my I phone 6 profile downloaded but when I go for install then restart after restart phone is again on12.5.4 version

The iPhone 6 can not be updated beyond iOS 12.5.4.