You cannot delete a certificate in DSM Control Panel > Security > Certificate because the Delete option in the context menu is unavailable.

If a certificate has been set as the default certificate, it cannot be deleted. To delete this certificate, set another certificate as the default certificate first. To do so, follow these steps:

1. Right-click on another certificate and select Edit.
2. Tick Set as default certificate and click OK.
3. Go back to the certificates list and select the certificate you wish to delete. Right-click and select Delete.