Go to Settings > General > Profiles. Select Apple Beta Profile, scroll down and hit delete, then yes on the confirmation. Just in case your iDevice already downloaded the iOS 9.1 update, check Settings > General > Usage > Manage Storage. If you see the iOS 9.1 beta there then select it and delete.
Then tomorrow, when the final iOS 9.0 is released, you'll be offered to update to it OTA.