Well, you've already integrated your X-Cart 5 with X-Payments so I think you know where your X-Payments is located. You need to login into X-Payments admin back-end and go to "Online stores", find your shop configuration there and change it as suggested.
Also, X-Payments user manual -
http://help.x-cart.com/index.php?title=X-Payments:User_manual#Online_Stores