Certificates can be cancelled as long as their status is "Draft" or "Pending Validation". To cancel a certificate, navigate to the certificates list and choose "Cancel Certificate" from the right-hand options menu. Once clicked, you will need to confirm your cancellation once more before it comes into effect.
Cancelling a certificate will free up a credit on your ZeroSSL account, which means that you will be able to replace your cancelled certificate with a new one.
Please note that cancelling an SSL certificate cannot be reversed.