Try these steps:
1.Install the latest version of iTunes.
2.Force your device to restart:
a)On iPhone 6s and earlier, press and hold both the Sleep/Wake and Home buttons for at least ten seconds, until you see the Apple logo.
b)Connect your device to your computer using a USB cable.
c)When iTunes asks you to update or restore, click Update (not Restore) to reinstall iOS and keep your personal data.
If you still need help, try these steps
If you can update, but your device starts up in recovery mode, follow the steps above again. On step 4, choose Restore instead of Update. After the restore completes, you need to set up your device from a backup.
If you continue to see an error during the update:
Check for updates to your Mac or PC. If you restart your computer for an update, check for updates again after you restart.
Try to restore your device using another USB cable.
Try to restore your device on another computer.