Apple ID /password

My grandmother passed away and my grandfather gave me her phone I've had it for a little over 6 months and even tho I've made a user name /Apple ID it will not let me take her off the phone because no one knew her password and we don't know what to we tried resetting phone and everything possible but nothing apple hangs up on me and at this point I am so anger I don't know if I even want to switch from my android can someone please help me I would gratefully appreciate it from the bottom of my heart