Do you want to talk about the "factory reset"? Yes, it's possible... but you will lose everything: in the end, your phone will be in the same state as it was when it left the factory.
No, it won't change anything since it's an official update, so for example, if you went from Android 7 to 8, you will remain on 8 even if you perform a hard reset.
Yes, it's possible... but you will lose everything: in the end, your phone will be in the same state as it was when it left the factory.