You can re-arrange the order of the vehicles in how they will display on the delivery screen by dragging and dropping them. The order from top to bottom is left to right on the delivery page. Once you have re-arranged them hit the "Save Order" button.
If you would like to add a new vehicle click the "new vehicle" button. If you are editing an existing vehicle click on the vehicle's line and then hit the "edit" button. Once you do either of those you will then see the following options. Once you have entered all the new information or changed anything then click the save button to solidify the changes.
(Click on image to enlarge)
All of the options here are completely optional for you to fill out other than the Title and checking the box for the Vehicle Position.