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:
Go to Settings > General, and tap Profiles & Device Managment.
Tap the iOS Beta Software Profile.
Tap Remove Profile, then restart your device.
When the next public version of iOS is available, install it to move back to a non-beta version of iOS. If an update is already available, you can install it by going to Settings > General > Software Update, but the iOS version must be later than the version you have.