You don't have to get " unbaptized". Baptism is all just faith and made up rituals anyway, and you can call yourself whatever you want, who is going to know but you ? Getting you wet and making a prayer over you does not make you who you are as a person. Once you dry off, you are the same person you were before.
If you want to become Buddhist, then read up on it, go to a Buddhist Temple and talk to people to see how you can officially convert, or just go to their Temple and participate in what interests you.
I dont know about baptism reversal but i do know that most if not all Buddhist sects will happily welcome ex-Catholics, and that all of my ex-Catholic friends just sort of grew distant from it without any particular event that unbaptized them. (Though a few of those ex-catholic friends still enjoy the iconography in limited ways.)