Hey, 

Which improvement are you talking about? If it's about intermediate, you should apply it in your college before the issued last date. You will be getting a form where you are supposed to fill up the details and mention which exams you are interested to rewrite.

If it's about B.tech, you should be aware about the declaration of the dates by management as they go unnoticed most of time. You'll have to apply for it your own college. 

Thank you and hope this helped :)