just go through the basics of every topic and start solving prev yr questions.I suggest you to start from these videos
LINEAR ALGEBRA
https://www.youtube.com/playlist?list=PLPiOSvowot1JLNguTkQUfXQJlLLML54yM
PROBABLITY
https://www.youtube.com/playlist?list=PLPiOSvowot1Lc6vs4tMzjTbUIVWVuuamb
SET THEORY
https://www.youtube.com/playlist?list=PLPiOSvowot1IRMhWIHuHQJTiLKiDpX1PJ