You should be able to just put it in recovery mode.

Link to instructions from Apple

1. Make sure that you're using the latest version of iTunes.
2. If iTunes is already open, close it. Connect your device to your computer and open iTunes. If you don't have a computer, borrow one from a friend (It actually says that! It also says apple store lol)
3. While your device is connected, force restart it with these steps:
4. On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold the Sleep/Wake and Home buttons at the same time. Don't release the buttons when you see the Apple logo. Keep holding both buttons until you see the recovery mode screen.
5. On an iPhone 7 or iPhone 7 Plus: Press and hold the Sleep/Wake and Volume Down buttons at the same time. Don't release the buttons when you see the Apple logo. Keep holding both buttons until you see the recovery mode screen.
6. When you see the option to Restore or Update, choose Update. iTunes will try to reinstall iOS without erasing your data. Wait while iTunes downloads the software for your device. If the download takes more than 15 minutes and your device exits recovery mode, let the download finish, then repeat these steps.