I cant reboot my iPhone 8 plus because it has full storage

Hi, I am trying to reboot my iPhone 8 plus for my younger brother that he can use it but he forgot his password on it and I was trying to reboot it for him and as I am trying to reboot it on Itunes it says that the device is full of storage and I try to fix it but I just can't get it to update to factory reset for him to use, so I was hoping that the apple supporters will help me out or give me advice on what to do? My email will be on the sound that I can respond asap please and thank you have a great day.