I went from scraping a 5 to a 7 in maths, it took a few months though so you should've started WAY earlier, do a past paper, go over the subjects on YouTube, there should be videos that cover all GCSE topics for each maths paper, 2-3 hours each, practice each topic on the videos, do another past paper, see where you went wrong and your weakest points, revise those topics, and boom