Try to update your device to iOS 11.2. After you tap Download and Install, the download will continue even if your device restarts. Wait for the update to complete.
If you can't update, turn off notifications for all the apps on your device, then update your device to iOS 11.2:
- Tap Settings > Notifications.
- Tap an app, then turn off Allow Notifications. Repeat this step for each app.
- Update your device to iOS 11.2.
- After updating, tap Settings > Notifications and turn Allow Notifications on again for each app.
You can also update your device using iTunes. Follow these steps:
- Open iTunes. Make sure that you have the latest version.
- Connect your device to your computer using the USB cable that it came with. Your connected device appears as an icon in the upper-left corner of the iTunes window. Learn what to do if you don't see the icon.
- Click the device icon.
- In the Summary tab, click Check for Update. After you click Download and Update, the download will continue even if your device restarts. Wait for the update to complete.
If you're unable to update or still have issues after the update, contact Apple Support.