After a relationship is created in a datamart there does not seem to be an option to edit it. Instead you have to delete it and create a new one if the wrong key was used.

Ideally, you could edit relationships same as in Power BI desktop. If Datamarts could use the same interface as the Power BI desktop relationship editor that would be great. If validation could be done on the type of relationship (1-1, 1-many, many-many) as is done in Power BI desktop that would be great.