Users who have upgraded to iOS 9 are now unable to downgrade back to the last version of iOS 8, iOS 8.4.1. Released in August, iOS 8.4.1 was a minor update that included bug fixes and Apple Music improvements. iOS 8.4.1 also broke the iOS 8.4 untethered jailbreak.
iOS 9 was first released to the public on September 16, and there have been two additional updates since that time. iOS 9.0.1 was released on September 23 and iOS 9.0.2 was released on September 30.