If you need to update your nameservers, please follow this tutorial
1) From your NivaCity account , click on Domains then select the domain name you want
2) On the next screen, select nameservers at the bottom of the page as per the attached picture
3) On the next page , select "Custom" and then enter your nameservers there under Nameserver 1 and Nameserver 2 , feel free to leave the nameserver IPs empty as those will be filled automatically. Upon DNS checks. then press save. 
4) If everything went well you will be presented with a blue successful Popup like the image below. 
If you get an error like Nameserver update has failed. Please see below for detailsNot authoritative, IP: xx.xx.xx.xxNot authoritative, IP: xx.xx.xx.xxPlease consult your domain registrar or nameserver provider for further assistance.
This means that the nameservers you entered are not authoritative for your domain name and you will need to make sure the web hosting account exists before you try to update the nameservers again.
NB : If you create the hosting account and still get the errors please wait at least 2 hours for the DNS to propagate before attempting to update as sometimes propagation takes a while.
If you are still having issues feel free to contact support