How can I stop using beta iOS on my iPhone?
To opt out of getting future betas, delete the iOS beta profile. On iOS 10, this can be done as follows: Settings > General > Profile > Delete Profile.

This will not remove the installed beta. It appears, the easiest way you can go back to a non-beta version is by restoring a backup. 

Some articles that may be useful: 


Removing iOS beta profile
Downgrading to iOS 9