Apple has posted a support document that explains what to do if the Slide to Upgrade screen won’t go away after you update to iOS 9.
How to fix persistent iOS 9 Slide to Upgrade screen:
1. Connect your iOS device to a computer that you’ve previously synced with iTunes. Then make sure iTunes is open.
2. 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.
3. 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.
MacDailyNews Take: Here’s one we haven’t seen on any of our devices that have been upgraded to iOS 9, have you?