When a customer wants to return the items in their order you can void the order by clicking the void button on the upper right of the order. This will enter the products within this order back into stock as if the order had not occurred.
This will not refund the customer.
If you also need to refund the customer, you must do so seperately. To learn about refunding the customer, please see the following article:
It is important to not click the red X icon next to products on the order, as it will cause discrepancies within your inventory. In circumstances such as this, it is best to manually adjust the Qty of the product in question to the correct amount and notate your records.
To learn how to process a return for a single product on a larger order, please see the following article: http://help.crystalcommerce.com/faqs-and-troubleshooting/how-to-use-crystalcommerce/accepting-returns-for-a-single-product-within-an-order