Refund

Most likely this isn't possible.

Only in case you haven't downloaded or opened the ticket yet, and haven’t loaded it into the app, you can contact customer service to help you with this. This can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook.

It is not possible to change the information on an already downloaded eticket, or to have it cancelled, since NS cannot invalidate etickets. You could have printed it already and still use it to travel. You could contact customer service in this case as well to see if they're willing to make an exception for you, but I wouldn't get my hopes up if I were you.