For each subject, go through the syllabus and highlight the topics that if you were sitting the exam tomorrow would make you lose sleep.
Then, write a revision plan so you go over these topics soon. If you don't understand something then ask a teacher or see if there is an explanation online.
If you struggle to remember things, aim to read the information three times, over a couple of days, and then try to answer questions or just write what you remember.
You do not have to remember everything. It is ok to get some things wrong in an exam.
Keep things in perspective. There are always chances to either resit exams or alternatively look for a path where practical skills can be developed.