1) if the phone is still fully operational, connect the iPhone to iTunes
2) connect the iPhone to a computer use iTunes to do a full backup -- If it asks if you want to transfer apps, say "no".
3) put the iPhone into [https://www.lifewire.com/using-iphone-dfu-mode-2000656|DFU mode] (should be a black screen, not a connect to iTunes symbol if done correctly), and allow iTunes to restore the iOS
4) Once it finishes restoring the iOS, iTunes will ask if you want to restore from the backup you just made. Simply agree to that, and you should be good to go.