This depends on how well you want to perform in your exams. If your aim is to just get passing marks, then it is fine to prepare from the last 5 years' question papers. But it also depends on luck as sometimes all the questions are new and none of them are repeated.

Therefore it is better to study everything and complete the syllabus and then practice from the last 5 years' question papers.

Try taking the guidance provided by your teachers and also your seniors. You can opt for coaching classes also to prepare better. Divide your strengths and weaknesses and work on them accordingly.

Lastly, practice from the previous years papers. This will familiarize you with the question pattern, the overall questions and give you an idea about the time limit.