MySQL Workbench: Edit hidden relationships in EER diagram
To my surprise I can't find any method to access the hidden relationship. It's listed in the table that defines the foreign key, but you cannot use this to open the relationship editor. However, by (temporarily) changing the foreign key the relationship is made visible again and you can then double click it to open the relationship editor. A context menu entry for the FK in the table editor and/or a small button to open that relationship editor would be nice. Open a feature request at http://bugs.mysql.com if you like.