When you joined to receive either iOS 10 public beta or developer beta, Apple installed a system profile on your iPhone so that you can receive OTA (Over-The-Air) updates whenever there is a beta release.
If you no longer want to use beta release, you will need to remove the profile in order to receive iOS public updates. On Settings app, go to General > Profile and look for “iOS Beta Software Profile”. Tap “Delete Profile” to remove it from your iPhone.
Now when you visit Settings > General > Software Update, iOS will only present you the latest public release. You might still see a beta version of iOS listed, which was downloaded before you remove the profile. To manually remove the downloaded iOS beta, go to Settings > General > Storage & iCloud Usage > Manage Storage. Look for “iOS xx.xx.xx” and remove it from the list.