Apple ID change for MAC

So my aunt gave me her laptop that was bought 4 years ago and I was trying to download stuff like apps and different browsers but it won't let me because it is still on her Apple ID and when I asked her for her password she told me she cant even remember anything about it even the email she used. So the question is how can I change it so i can use my own ID and password, like a total reset?

called apple too but they said they needed to talk to my aunt and she's halfway around the world!!!

k thanks. need help so bad