Flexible fares have less or no cancellation fees than non-flexible fares. Generally speaking, non-flex fares cannot be cancelled.
The specific terms and conditions for the booking you have made are explained for each ticket in My NSI.
If you have bought your ticket on NSInternational.com or via NS International Customer service, follow the step-by-step instructions on Change or cancel your trip to cancel your ticket yourself.
If you can cancel your ticket online, but instead ask NS International Customer service or a NS International Service desk to do this for you, you pay a service fee.
In all other cases, contact the original point of sale.