If you can't get it done in Mijn NS, then you should contact customer service directly for this. You can contact them by phone 24/7 at 030-751 51 55. You may also try to contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook.

Hey Fatih, welcome to the NS Community.

Tochjo has given the correct answer here. You can use one of the following methods to contact us so we can help you with this. At the same time, we can look into why you can't adjust your product with your Mijn NS account! 

Simply go to the Webshop and order Weekend Vrij.