When your device gets into an endless reboot loop or keeps restarting, or freezes at Slide to Upgrade screen after update to iOS 9, you can try to do a hard reset, force reboot iDevice with free iOS 9 device reboot software, factory reset your device or do a system repair. Below are the details. These 4 methods can be applied to iPhone 6s/6s Plus/6 Plus/6/5s/5c/5/4s, iPad 4/3/2, iPad mini 4/3/2/mini, iPad Air 2/Air, iPad Pro and iPod.

Way 1: Hard Reset

If the Power button and the Home button on your iPhone are still workable, the hard reset comes in handy. Press and hold both the Sleep/Wake and Home buttons for at least 10 seconds, until you see the Apple logo. This should get you out of iOS 8 reboot loop.

Way 2: Use Free ReiBoot to Get Out of Bootloop without Restoring

Free tool ReiBoot (Mac) is powerful to help get out of boot loop problem in iOS update or iOS jailbreak. It forces reboot your iPhone, iPad, iPod and make them back to normal without data loss. Download and install this freeware on your Mac (OS X 10.11 El Capitan, OS X 10.10 Yosemite, 10.9 Mavericks, 10.8 and lower). If you are running Window 10/8.1/8/7/XP/Vista, free download ReiBoot for Windows.

Connect iPhone to computer, and run ReiBoot (Mac). The program will detect your device automatically.
Click on "Enter Recovery Mode". ReiBoot will remind you if your device is already in recovery mode.
Click on "Exit Recovery Mode" and your device will boot up.

Up to 99% users said ReiBoot have successfully fixed their iPhone, iPad, iPod iOS 9 reboot loop. Besides reboot loop, ReiBoot also fixes iOS 9/8 devices not responding, and solves iOS 9/8 red iTunes logo stuck problem.

Way 3: Restore iOS 9/8 Devices to Factory Settings

Factory reset will erase all your data. Backing up your device before you start. Connect your device with computer and then iTunes will detect your iPhone in recovery mode and restore it to factory setting. However, make sure you've backed up iPhone to compute or iTunes before factory reset.

Way 4: Repair iPhone Operating System with iPhone Care Pro

After trying all methods above, your device still stuck on boot loop? Your repair the corruptediOS operating system to get youriPhone reboot to normal state with Tenorshare iPhone Care Pro and Tenorshare iPhone Care Pro (Mac). It will fix missing or corrupted system-related files by reinstalling the iOS firmware and get your iOS device normally.

Note: After the repair, your iOS device will automaticlly update to the latest iOS version.
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See More: 4 Methods to Fix iPhone/iPad/iPod Stuck in Reboot Loop