Open Crmble and go to the contact menu. Search for the contact and click the trashcan icon to delete it. If the trashcan icon is not active, it’s because you can only delete contacts that are not linked to any card of any board in your Crmble Team. You can check on the right column of the contact section the list of leads a contact is linked with. You can replace the main contact of a lead using the "Switch" that you can access by clicking the pencil icon to edit the contact from the back of the Trello card.