Try to do at least one Problem extra (which you were not able to do in contest) after the contest. After trying your best, read editorials and see the prerequisites. This way after every long challenge you’ll learn new topics. In Long Challenges, ideally no Problem should be unread. There maybe some Problems which require hard topics (which you might don’t know) but there subtasks. You can try to solve them partially. The best way to learn new topics is by practicing problems. You can check past Long Challenges and look at the new topics and try to learn them. If you are wondering when to see editorials of problems which are unable to solve read this.