I have given my improvement exam from CBSE this year but this year my marks in maths is less than previous year
If you have given the improvement exam, then the best out of two will be considered, but if you have re-appeared for board exams, then the latest marks will be considered. Make sure that the exam you applied for was indeed an improvement exam form and not just board examination form. ANd it is recommended that you contact CBSE through e-mail regarding your query because the guidelines I've mentioned might change this year.