iOS 9 Update Failed in iPhone & iPad "Software update failed. An error occurred downloading iOS 9" - weird message after clicking 'Download and install'. Restarting devices didn't help. What's the problem?
iOS 9 just came out and since many are trying to update to it, it seems like a server overload on Apple's part, this article from 9to5Mac also confirms this. Unfortunately, all you can do is be patient. I still only see the iOS 8.4.1 update available on my iPhone 5S.