If you have an iPhone with Touch ID security you'll be prompted to scan one finger during the setup process. You can skip this step if you like.
It takes about five minutes to walk through the process and the only information you'll need is your Apple ID and password and the password for your Wi-Fi network, if one is necessary.
Starting over via Wi-Fi
iTunes is just one way to set up the new OS. The other is to switch on your device, connect to a Wi-Fi network, and then start the setup process. When you do, you'll arrive at a Set Up screen where you can choose to set up the device as a new one, restore it from an iCloud backup, or restore it from an iTunes backup. If you choose the last option, you'll be prompted to connect your device to your computer. We'll examine this option first.
Restore from iTunes backup
If you choose to restore from an iTunes backup, connect the device to your computer, select it in iTunes, and from the Restore from this backup pop-up menu choose the latest backup for your device and click Continue. Your device will display the Apple logo and a progress bar, indicating that it's being restored from the backup. When that's complete the screen displays an Update Completed message. Tap Continue and you'll be walked through a few steps — enabling location services, choosing accounts to use with iMessage and FaceTime, creating and confirming a passcode, and choosing to share analytics data with third-party developers.
When the update completes, iTunes will copy your backed-up apps and data to the device. Depending on how many apps you have, this could take many minutes.
Restore from iCloud backup
If you've backed up your iOS device to iCloud (rather than to your computer via iTunes), select Restore from iCloud Backup on the Set Up screen. You'll be prompted for your Apple ID and password. Enter them and tap Next. Your device will sign into iCloud and you'll be presented with the Terms and Conditions screen where you tap Agree twice.
Choose the desired backup (you may have more than one depending on the number of iOS devices you've backed up), and then wait; the process can take anywhere from a few minutes to many hours, depending on how many apps you had installed and how much data you had stored in those apps. (As mentioned earlier, restoring from an iTunes backup is much faster than restoring from iCloud, which is why many people back up to iTunes periodically even if they normally back up to iCloud.)
When the restore is finished, your device restarts, and you see a screen explaining that you have a few more steps to follow. These steps include entering your Apple ID's password again, selecting which identities you want to use for iMessage and FaceTime, creating a passcode, and setting up iCloud Keychain.
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