While some iPad and iPhone users want to download the iOS 9 update today from an older version, like iOS 8.4, others need to try and upgrade from the newer iOS 9.1 public beta. If anything, this is more of a downgrade but not in the sense of stable releases.
We already touched on downgrading from iOS 9 to iOS 8.4.1. Those running the latest beta build will be wondering how to update to iOS 9 today, but in this situation from the iOS 9.1 public beta that released last week. This exact question had been raised on the official Apple developer forums and considering there’s well over 10 million devs, this could be a common situation for those running a public beta on an everyday device.
How to upgrade to the iOS 9 update today from iOS 9.1 beta 1? In a nutshell, the upgrade message won’t appear on devices today that run iOS 9.1 public beta. Instead, you will need to restore your device through Apple iTunes and this will wipe all user data from the iPad, iPhone, or iPod touch.
You can only restore your data if you took a backup with an earlier version, so any iOS 9.1 beta backup cannot be restored to your iOS 9 public install that will release on Sept 16, 2015. You either have to start with a clean install, or restore an older backup from before upgrading to the iOS 9.1 beta.
The key point is to hope you have a backup from the iOS 9 GM seed or another recent backup before updating to iOS 9.1.