Unfortunately at this time, orders are not able to be edited or altered after they are placed. However, you are able to potentially cancel the item before it ships.
After placing an order on Rivly, if you decide that you'd like to cancel the order, there will be a "Cancel Order" button that appears for orders that are eligible before the item has shipped. Clicking this button will send an immediate message to the seller requesting cancelation.
The seller at that time is able to cancel the order and provide a full refund if the order is caught before it has shipped. Unfortunately, there is no guarantee that the order is able to be canceled by the seller depending on the shipment status.
If all else fails and the item has already been shipped, you must wait until you receive the item before deciding to initiate a return. For further information on how to start a return, see the information in the link provided.