I am using the latest iOS (as of Nov 21, 2020), it's 14.2 (stable version). But the beta upgrade keeps poping out asking me to download it. It's really annoying.
How do I opt this out on my iPhone?
Thanks,
First, you can try deleting the Beta Profile:
Remove Profile
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.