You may use either the "edit" link on one of the nodes/boxes of the person in question or use the "Edit Profile" link on their profile page. You can then use the 'Relationships' tab to update the marital status and/or marriage order between those people.
As a reminder, Marriages are displayed per marriage order-not by marriage dates. Therefore, the most recent marriage will be displayed directly next to the person, then the one before that one, finally the showing the first union furthermost away.
Also, another factor to keep in mind is the perspective of which you are viewing these said unions. For instance, when you login and view Person A's Tree you will note A is directly to the most recent spouse, then the one they were married to before that one, etc.