Yes, you will be eligible to appear for JEE Mains and Advanced 2025 if you reappear for your 12th board exams in 2025 to improve your marks and meet the 75% eligibility criteria.

Here is the eligibility criteria for JEE Mains and Advanced your reference:

• JEE Mains : Candidates must have passed their 12th examination or equivalent in the previous two years (2024 or 2025 for the 2025 exam).
  
  
• JEE Advanced : Candidates must be within the top 2,50,000 rank holders in JEE Mains and meet the 75% aggregate marks in the 12th boards (or be in the top 20 percentile in their respective boards).
  
  
  I hope this answer helps you. If you have more queries then feel free to share your questions with us we will be happy to assist you.
  
  Thank you and wishing you all the best for your bright future.