A select number of users trying to update to iOS 9 have been reporting on a thread at Apple Support Communities that their update has been stuck at ‘Slide to Upgrade’ screen. While the issue is not widespread, it appears that the only way to fix the bug is to restore your iOS device from an iTunes backup.
Connect your iOS device to your computer and open iTunes.
Select your device. If you don’t see it listed, force restart your device by pressing and holding both the Sleep/Wake and Home button for at least 10 seconds, until you see the Apple logo.
If you made an iTunes backup of your device before updating to iOS 9, restore your device from that backup. If not, you need to make a backup of your device in iTunes, then restore from that backup.
Make sure that you have create a backup of your device in iTunes from which you need to restore. A backup in iCloud won’t help in this case.
Let us know if this resolves the issue for you in the comments section.