Grade 3 to grade 6? Watch
The issue with maths is that it can be difficult to revise as they aren't really straight facts. Your best bet for maths is to look through your paper with a teacher and identify your weakest areas, then do lots and lots of practice questions (make sure you track how many you've done, with a mini spreadsheet or something similar, it will help especially towards the end to give you a bit of a confidence boost) after doing a certain amount of practice questions (this may be all the ones in your textbook, the CGP can be good for this especially the ten-minute quiz books I've put a link at the bottom) do another practice paper, provided you put the work in you should be able to achieve what you need.
Best of luck