Hello Aspirant, It is difficult to change OBC certificate but you can contact to CAT Helpdesk.

Candidate Helpdesk is available by phone, Monday to Saturday from 9:00 a.m. To 6:00 p.m. The CAT Candidate Helpdesk is closed on Sundays and on National/Public holidays. Candidates can contact the CAT Helpdesk:

Toll-free: 18002090830
Email: cathelpdesk@iimcat.ac.in (mailto:cathelpdesk@iimcat.ac.in)

I hope this will help you.