I'm going to tell my grandma to fuck off.
You can't be serious. Why would say something like that to your grandmother?
Because she sent me a terrible birthday present, it's really rude of her.
It's really rude to talk to your grandmother like that. Even if you don't like the present, you should treat her with respect and thank her for it.
I don't care, she's old. I'm always going to be in the right regardless of what I do.
I don't know why her age has anything to do with you saying something so rude to her. Getting a bad gift is no reason to curse at your own grandmother.