1:Make sure iTunes doesn’t automatically sync with iPhone when connected.
2:Connect your iPhone to iTunes and Back up by selecting Preferences > Backup from iTunes, or right-click on your iPhone and choose back up.
3:Restore your iPhone by selecting Restore under Preferences.
Note that restoring the iPhone will restore the previous iPhone firmware. Your iPhone will be completely erased after the new firmware is installed. So make sure your back up is good!