When one is eligible for an SSL Certificate refund?

Sectigo offers a 30-day return policy.
Once the 30-day refund period has expired, no refunds could be provided.

This is only applicable for new orders and renewals. This is not applicable in the first 30 days following a reissue of an existing certificate.
Note: A 2nd year subscription ssl activation is a reissue and not a renewal.

How to receive a refund?

This is a fully automated process. You can cancel your SSL via the SSl panel. A message will pop-up, to inform if the order will be refunded or not. The refund will be processed within 24 hours (This is not a not real-time operation).

Please see this tutorial how to delete the certificate.