By taking them as a private candidate and paying for the exams yourself (or getting your parents to pay for them).
However I really wouldn't bother if I were you. What's the point? Do you plan on going to uni early? In which case, you'll have no social life as everyone will be going to bars and clubs that you won't be able to get into, and how exactly are you going to live away from your parents at the age of 15? Not to mention not having much in common with the other uni students.
Or are you just wanting to take them early and then sit around for 3 years doing nothing? That's not going to really work either, as you'll still have to be studying something even if it's not for the sake of taking exams.
All in all it just seems like a bad idea.