"How to downgrade to iOS 9 on your iPhone or iPad," if you choose "Update" instead of "Restore and Update," it will replace your 9.1 beta OS with the official 9.0 release OS, without deleting any of your user data! In other words, no time-consuming restore from backups are necessary. Once your phone completes the update, all of your data is still in place.
I’m not sure when this new option showed up, but it’s real, and it’s spectacular.
And afterward, if you no longer want to be part of the beta program, or prompted to install the 9.1 beta, delete the profile from your phone: Settings – General – Profile – Select “iOS 9 beta software profile” – choose delete profile.