It is possible to increase your grades. I got ABB at as, and AAB at A2 back in 2005.
It's hard though. Not only did I have to do better in harder exams, but I had to resit old exams whilst trying to learn new material.
Most people do not improve their grade. Most do the opposite.
I was able to improve because I identified what I was doing poorly. My revision was slapdash and I was lazy in lectures, then I wasn't. If something doesn't change then your grades wont.