Now according to reports, it seems that there are users who are affected by an unresponsive “Slide to Upgrade” screen after upgrading their phone to iOS 9. Basically this is kind of like the last bit of the upgrade where you will need to set up your phone again, but user reports are suggesting that they’re stuck at this screen and can’t do anything about it.
This issue has been reported on Apple’s support community as well as the MacRumors forums. However it seems that this is an issue Apple is aware of and has since published a support document telling users what to do in this situation. Unfortunately for users, this “fix” will see you restoring your phone to its last backup, and you will have to start the process all over again.
If you have yet to download iOS 9 and if you haven’t backed up your iPhone yet, you probably should go ahead and do that before updating. This issue does not seem to be a widespread problem so hopefully it won’t affect you when you update.