For this, you need to contact customer service. 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. Note that it can take a few days to receive a reply if you contact them online.

Because a birthdate doesn't change you cannot do that yourself. You have to contact the customer service and they will ask you for an explanation why you want to change it.