This is way harder than Apple support page
https://support.apple.com/en-us/HT201263
If this hadn’t happened since iOS 4.3, I’d be tempted to see which is right
After starting iTunes, then plugging in phone:
- On iPhone 8 and later: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the connect to iTunes screen.
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 the connect to iTunes screen, let the download finish, then repeat step 3.