How can I edit an alias to make it default mail?

How can I edit an alias to make it default mail?

I keep getting message that my alias "does not have the domain based mail alias as default.Choose a mail alias as default." Yet I cannot find any way to make that change. I've tried to delete the existing alias and start over, but the Zoho will not permit that. Have tried through Control Panel and everything I can think of, but am blocked from deleting or editing. How can I make my alias the default?