how to reset my i phone without delet

my sister gave me a phone and our phones are connected and i’ve been trying to make a new apple id and for some reason it won’t work and i can’t download anything without it asking for a apple id i used her apple id for the phone and her password but for some reason it says it’s wrong and it won’t work so i made a new account and it still won’t work so i have no clue on what to do and i want to factory reset to see if it’ll do anything but it will probably delete her pictures and information from her phone she has now