To unassign using the Driver Status button navigate to the Dispatch screen.
Select the driver that you want to unassign from, the "Driver Status" button will be come available. Click on Driver Status.
The driver status window will appear with a list of orders assigned, and a list of options. Click on the order you want to unassign. Once it is highlighted click on the "Unassign Delivery" button.
When you're done click the "Close" button on the driver's status. You will now see in the main screen that the order is unassigned and is pending delivery.