iPhone 4s iOS 9 Release: 10 Things to Keep in Mind

You'll Be Able to Downgrade to iOS 8

Keep in mind, you should be able to downgrade back to iOS 8.4.1 in the week or two after the iOS 9 release date. Nothing older than iOS 8.4.1 though.

Apple should continue signing off on the iOS 8.4.1 update for one to two weeks after the iOS 9 update's arrival. If it does, you'll be able to take advantage of a loop hole that will allow you to downgrade back to iOS 8. 

If you aren't familiar with the process, get familiar with it right now because it could come in handy if iOS 9 doesn't live up to your expectations.