Sure, you can take any exam as many times as you'd like. I'm not sure of your school's specific policies on that, but I doubt there would be any issue. If you're concerned, just ask your guidance counselor.
Calc AB is easy and straightforward. I'm not going to go over the specific differences and extra content found in BC, because you can find a vast wealth of that information online already. Rather, just be sure you're aware that the extra material in BC (equivalent to a whole other semester of calculus at most colleges) is tougher. You're going to have to work -- it isn't easy. Unless you're breezing through AB, or have some incredible passion for calculus, it may not be worth it.
Also, remember that the BC exam includes an AB subscore. So, let's say you took the exam with barely any knowledge of the additional BC material. You would probably get a 2 or so on the exam, but assuming you have a good grasp on the AB material, you can still get an AB subscore of 5, just as if you had taken the AB exam alone. The BC exam is designed this way.