https://support.apple.com/en-us/HT202033

https://support.apple.com/en-us/HT208109

Make sure your device is ready for service by completing the following steps: • Back up your data to iCloud or iTunes • Turn off Find my iPhone Choose a back up method Use iCloud to back up your iOS devices if you want an automatic backup solution, you want to restore content to your device from almost anywhere using Wi-Fi, you don’t connect your device to a computer very often, or you don't own a computer. Use iTunes if you frequently use the computer that hosts your backups, you don't have an iCloud account or don't want to use iCloud, you want on-site and networked backups, or you'd like to have a manual or secondary backup to use with your iCloud back up. For more information on back up options, go to support.apple.com/HT204136.