How/what should I revise for the A level History exam ?

Revision is extremely individual, my recommendation is that you figure out how you learn sooner rather than later. Do you learn by making mind maps? Re-writing your notes? Verbal testing? Timelines? These are all a few examples of how you can learn effectively.In terms of the history exam if you are aiming for an A/B it is essentially about what I call "box ticking". Get acquainted with the syllabus early on, really know what the examiners are looking for in terms of AO1, AO2 and AO3, doing this well can get you a high A. So know a plethora of examples, make sure that your essay style flows/your points connect and have a short concise introduction and conclusion. To get an A* you need to do all of the above but also go that extra step to show evidence of individual thinking, I call the Cherry analysis.