A domain name cannot be registered without a valid nameservers, you can assign nameservers when you are placing order for domain registration only. The system assigns a nameservers automatic if you are buying a full package here of hosting and domain.
For resellers and those that have a hosting somewhere, you can assign your nameservers or update a new nameservers from your client area here.

The process is instructed bellow:

Login to the Client Area > Domains >> My Domains >>> Manage Domains >>>> Nameservers >>>>>

You have two columns of nameservers with the existing nameservers, delete and type the new nameservers.
Save chnages and the new nameserver will take effect after propagation.