iPhone 4s iOS 9 Update: 10 Things to Know After Three Weeks

You Can't Downgrade to iOS 8

If you're coming from iOS 8, know that you can no longer downgrade once you make the leap to iOS 9. You're stuck with iOS 9.0.2 or iOS 9.0.1. 

Apple is still signing off on iOS 9.0.1 but it could close up the downgrade loophole at any time. If you're not satisifed with iOS 9.0.2, you might want to think about dropping back down to iOS 9.0.1.