If you want to change the order of clients within your deal, first step is going to edit deal (pen icon), from deal view.
Next to every client, you have delete button. Once clicked on recycle bin icon next to 1st client in your deal, secondary client will become first (primary client).
Then, click on “add another contact” button, and in field first name type first few letters of clients name, and choose it from drop-down menu. (that way you`ll add client you have previously deleted, without creating duplicated contact).