What should I do about making up 20 days from the previous Ramadan, knowing that there are 43 days left until the next Ramadan, and my exams are in 40 days, and should I fast them now or postpone them until after Ramadan?

The missed fasts of Ramadan (definition) must be made up before the advent of the next Ramadan. Delaying the make-up fasts without a valid excuse is considered a sin, and it necessitates feeding a poor person for each day that the make-up was delayed. Preparing for exams is not considered an excuse for abandoning the make-up fasts; one can reconcile between them by managing time effectively.