You Can Downgrade to iOS 9.0 or iOS 8.4.1
If you're already on iOS 9.0.1, it's worth knowing that you can, for the moment, drop back down to one of two older iOS updates. This lets you trial the iOS 9.0.1 update to see how it performs on your version of the iPhone 4s.
Apple is still signing off on iOS 8.4.1 and iOS 9.0 which means that you can use a loophole to downgrade to either one of them. You can't drop back down to anything older than those though.
We've put together a guide that will help you downgrade if you need to. Apple typically closes the loophole up after a week or two so you'll need to move quickly.
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