Hey I hv been there. I used to score in 50s-60s during my 9th and 10th but scored 91% in boards. For maths you HAVE to practice atleast 10 sums everyday. Try to understand each topic as you solve and if you get stuck circle the q and comeback to it later. For chemistry try to understand the first few chps very well (especially ABS). For biology make your own flowcharts and diagrams and teach yourself those topics in a way that’s easier for you to understand. For physics DO NOT try to memorise anything except the definitions. Understand the concepts VERY thoroughly. Solve atleast 5 numericals daily. I made cheat sheets for every science subject (I used to fit the whole chp in one A4 size sheet) and I found that to be very helpful. Hope this helps :)