Can't remove Apple Account from my Grandma's iPhone to use it

so my grandma gifted me my grandpas phone who passed away. She tried to factory reset it and erase everything she couldn't she then sent it to us. She even tried to changed the Apple id password and that didn't work. So now I tried to put the phone into recovery mode and now it when i am trying to set up the phone it is saying that it is still linked to my grandpas apple id account and i dont know what to do.

[Re-Titled by Moderator]