Unfortunately there is no way to change your cast after the Joint Entrance Examination has concluded. In order to get benefit of any kind of reservation, you have to fill your cast and related information at the time of filling the form, as there is no such option to do so after the declaration of results.

You can also use our college predictor to see your chances in all the colleges:
https://engineering.careers360.com/jee-main-college-predictor?icn=QnA&ici=qna_answer

Hope I have answered your question. All the best!