How to change caste from General to OBC

Sir I had made mistake during my 10th class registration that i had told my caste as General but I'm in OBC category and it was continued till my PG. But now I'm preparing some competitive exam so in that I want claim reservation under OBC so I need OBC certificate. So please sir help me through what legal processure i can change it. Please help me. I'm awaiting good reply from this forum. Thank you.