The DWT value on the shipping label is coming from the value entered for dimension of the package you are using to ship the order. This can be found under the Packages tab of the Item Fulfillment under the Length, Width and Height fields.
The value that will default on the fields will depend upon the package selected for the shipment. Please note that they are just default values. If the dimension is incorrectly entered you can still correct or change it on the Packages tab before saving the Item Fulfillment.
Once the Item Fulfillment has already been saved and the Tracking Number has already been generated, it cannot be changed anymore.