If you were unable to deliver any items during the route, then after you completed all the tasks, before returning to the depot, you will be suggested to complete the returns. You will see the Return Stop tiles with the reverse arrow, suggesting you to swipe back to the Delivery tile to re-attempt the delivery. If you are not able to re-attempt, then expand the Return Stop tile and verify that the delivery address here is the addres of the depot by default. Tap Delivery, and in the Return to field specify the name of the person at the depot and optionally add a driver note. Tap Mark as returned below.
You can get proof of the return by using the barcode scanner, getting a signature, or taking a photograph.
Tapping Mark as returned closes the Complete return screen.