How do I opt out the Apple iOS beta program?
First, you can try deleting the Beta Profile:

Go to Settings &gt; General &gt; Profiles
Click Remove Profile
Restart your device

If you still receive update notification, you should restore your iPhone with the latest iOS release.
In that case you will be sure that the beta profile is completely disabled and won't send you any beta update popup anymore.
Some articles suggest to change the device's calendar date to avoid popup alerts and beta updates, I don't recommend this solution.