Hi

The recent update by JEE advanced officials for

Candidates who are not able to upload a valid category certificate (for ALL categories) during registration, can upload a self-declaration in lieu of the same, and must upload the valid certificate at the time of reporting during JoSAA.

you are supposed to upload it at the time of registration of Josaa.

all the updates by them you can check at their official website

https://jeeadv.ac.in/

hope this helps!