Today Apple released iOS 9.0.1, the first bug fix update for iOS 9. Included is a fix for the problem where a small number of people’s iPhones froze during the last step of the upgrade, forcing them to restore their iPhone from a backup. This bug seemed to affect mostly people jumping straight from iOS 7 to iOS 9.
To update, connect your iPhone, iPad, or iPod Touch to WiFi and head to Settings > General > Software Update.