To modify an order you first need to go the New Orders page. From there you can change what type of parcel you want to send by clicking on the drop-down menu in the Type column. You can also edit parameters like dimensions and weight from their respective columns.
To edit other parameters click the blue button with the down arrow to the right of the parcel and click the Modify option.
From there you can change the other parameters of the order. Then click save to confirm you changes.