Wait a Few Hours to Install
All eyes are on iOS 9 and its release date.
When Apple releases iOS 9, it will release it for everyone all at once. We typically see Apple's servers get slammed with requests right at the iOS release time and we expect the same thing from the iOS 9 release time.
This usually leads to prolonged download times, something that could extend your installation time past the one hour mark. If you don't want to deal with long download times or any of the other nonsense that typically accompanies these big iOS releases, wait a few hours.
These initial server and download issues typically dissipate after a few hours. And once they do, you should be able to install iOS 9 without any hassles.