To add or edit a marraige:
Click on one of the partners
Click on the "Partners" tab on the left sidebar. Note that if they don't have a partner yet, you have to add the partner first with the "Add partner/ex" button.
Click on "Edit partnership details"
Change the partnership type and add a date and location if desired. then click Okay.
Family Echo doesn't have an option to display partnership dates on the family tree. This is probably because there's not a great way to display the data in situations with multiple partners. But I'm just guessing.