How do I become unbaptized so I can be a full fledged Atheist?

A baptism only means what you make it mean. To a church it may mean being saved by God. But if you don't want that, then just look at it as you being dunked in water and that's all it means. By the way, if want to be an Atheist why do you care about going to Heaven and being saved? You may want to really think about the whole atheist thing. If you're becoming an atheist so you don't have to be Christian, you should rethink your beliefs. If you don't want to be Christian, then don't be. You don't need a replacement as a reason why you are not Christian.