If someone wants to change his/her birthday that's very often a sign of fraud because that never changes. Such a request is therefore only possible by contacting the customer service. Be aware that they will ask you for an explanation.

Hello Nima dherpa and welcome at our community,

I would like to advice you to reach our customer service. My colleagues can change the date of birth for you.

Recently (months ago), NS changed some things in their systems so the date of birth was not needed anymore for some processes.

This decision was later reversed, after which the dates were put back in the system but wrongly (or missing comepletely!). Contact NS Customer Service to have it sorted out.

@Tamzin there is no fraud here, it was a mistake caused by the NS IT department. Many others have stumbled upon the same problem (why did NS delete or change my date of birth?)

I was just explaining why you cannot change the date yourself.