I think this repair guides would probably be the best for you. iPhone 7 Volume Control Buttons Replacement I'm not too sure about the iPhone 8, although I would assume it would be similar to the iPhone 7.
This is the best way I can think of. 1. Open the Settings app and go to “General” 2. Choose “Storage & iCloud Usage” 3. Go to “Manage Storage” 4. Locate the iOS software update that is nagging you and tap on it 5. Tap on “Delete Update” and confirm that you want to delete the update* This deletes the available iOS update which stops the iOS update from popping up every day, however, the moment you’re on a sustained wi-fi connection for a while the iOS update will download itself again automatically and start sending pop-ups to install it again. If this happens, just redo the steps. You won’t lose the ability to install the iOS update, you can download it again yourself by going to Settings > Software Update, or just join wi-fi and leave the device unattended and it will download automatically without prompting. I hope this helps!
