NB! Your domain must be active in "My Account", in order to update the Name Servers.
1.) Click on the link provided:
https://www.registerdomain.co.za/clientarea.php?action=domains
2.) Search the Domain name in question.
3.) Select "Manage"
4.) Now select "Nameservers"
5.) Remove existing name servers, enter your new desired nameservers and select "Change Nameservers".