Hi

According to the their official website the refund process had already started

"Notice to candidates: NEW

For multiple payments, the refund process has started and the money would be refunded in due course of time. Some transcations might take longer time to settle, since they require manual settlement between the SBI-backed payment portal of JEE (Advanced) 2021 and payment service portals of individual candidates.

They already had a link for issues related to payment of fees which has been deactivated at present.

so you need to get in touch with them through their help center

https://jeeadv.ac.in/contact.php

Hope this answer your query!