You can update nameserver information by contacting your registrar. Some registrars offer an online system through which you can directly access and update your information. Otherwise, you can make these changes by contacting the registrar's customer service.
Remember that your domain must be associated with your Membership ID (token) in order for traffic to resolve to your nameservers, so when setting or confirming your name servers at your registrar, also confirm that your domains are associated to your Membership ID (token).