Hii,

The Caste certificate usually holds a validity o f 1 year. So to retain your proof of category you need to keep on updating it every year. As a far as your question is concerned, No you can't submit the NC OBC certificate of 2015  in the CAT 2020 form. You need to renew your certificate from the issuing authority near you place and then upload it. For further help you can contact CAT helpdesk numbers available on their official website.

https://iimcat.ac.in/per/g01/pub/756/ASM/WebPortal/1/index.html?756@@1@@1

All the Best.

Thank You.