Dear candidate first of all you need to sign in to the registration page of jee advanced by visiting on this website https://jeeadv.nic.in and by entering your details your candidate portal will get open then click on the uploaded documents option there you will get an option for "Edit/ Change Uploaded Document" click on that then you will get many option to change certificates such as 12th Marksheet, 10th Marksheet and category certificate. You need to select Category option from it then upload your OBC-NCL certificate. Remember to do this procedure fast as the last date is 2nd October till 5:00pm.

Hope this would help!