# Connect your iPhone to a computer that you have previously synced with iTunes, make sure iTunes is open.
# 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.
(P.S: These steps are from Apple inc.)