Delete iOS 12 beta
How do I keep from getting the update message from appearing on my iPhone for iOS 12 beta?
iPhone 6s
iOS 12 Beta cannot be discussed in the public forums as per the terms you agreed to when installing it. Its considered confidential.
If you wish to uninstall it see here first: https://beta.apple.com/sp/betaprogram/unenroll#ios
And then here: How to install or uninstall iOS beta software - Apple Support
For any other issues with the beta, us the Feedback Assistant app and let Apple know about it as you were asked to do when joining the Beta.