You Can't Downgrade
If you already downloaded iOS 9.0.1 on your iPhone 6s or iPhone 6s Plus and you wish you could go back to iOS 9.0, know that you can't. You're stuck on iOS 9.0.1 until Apple rolls out its next iOS 9 update.
Apple isn't signing on the iOS 9.0 updates for iPhone 6s and iPhone 6s Plus which means that the downgrade loophole is closed.
While you can't downgrade, now would be an excellent time to get familiar with the downgrade process so that you'll be able to downgrade in the future.
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