Jitendra, once you fill in your NEET application form, the last chance to make any corrections to your application form is the correction window. Once the correction window has been closed, you cannot make any changes whatsoever. NTA has been clear about it. NTA neither makes the changes on your behalf nor does it make any changes to the application form itself. The NEET merit list is prepared based on your NEET score and the Category you belong to. So, your Category cannot be changed after NEET results have been declared. If you belong to OBC, during Document Verification and admission, you will have to submit the proof of your Category. Failing which you will not be considered under General Category but your admission will stand cancelled. So just to answer your question, you cannot change your Category once the results have been declared. The correction window is the last chance at it.