How do I issue a discount/credit on an open order?

Once an order is submitted, you cannot change the price of individual items. You can, however, issue a credit or discount on the order total. To do this, you simply have to visit My Orders Received and enter the credit/discount in line with the order in the "Credit" section. 
Please note that you are still charged a BrickLink fee based on the original order total before any credits or discounts are applied.