It's the most simple advice but it's integral in improving your grade. I assume that you get to see your exams so you see what you do well and don't do well in, no? Well go over them and look for patterns of what you're good at and what you're not. Don't necessarily forget about the stuff that you're good at, but focus on learning the stuff that you don't because that's probably what is preventing you from improving overall. Use resources that you know work for you.