Well for starters you should probably start revising then. I'm not gonna give you the cliché techniques, but rather I'd recommend using the CGP textbook in conjunction with your notes for the sciences. I find that for biology, it's good to try and link the topics together (e.g. enzymes to respiration) and make I meet what's required on the specification. For the other core subjects (physics and chemistry included) there's no simple way other than to go through all the content and learn it. Now, be realistic as well. It's almost impossible, I think, to pull an A* if you've consistently gotten bad grades since year 9 or below, though an A or B is more plausible with the right attitude. To get beyond this, you'd need to spend a lot of time actually understanding the subject, but I feel as if this option is no longer there, especially as you've probs finished the majority of topics for all subjects.