You can cancel your order from the ‘Order details’ screen before it is out for delivery. 

You can do so by clicking on the Account icon on the bottom of the App Home page > select "Orders" > Select the order > Click on “view details” for the shipment which you want to cancel > tap “Cancel”. (Refer Image)

Once the order is shipped, you can refuse it at the time of delivery and a refund will be processed into the payment source, if the order amount was paid online.