You can cancel their order before it has been shipped. To do so, simply visit the order details page, where you'll find the option to cancel. This feature is available until the shipment is booked.
Alternatively, if the order has already shipped, you can refuse it at the time of delivery.
Please note, we currently do not offer an option to modify orders. You can either cancel the existing order and place a new one, or reach out to our customer support for assistance with any changes.