Changing nameserver in your domain is simple process. Please follow the instructions below.
Step One: Go to Domains >> My Domains menu.
Step Two: Click on the domain name you want to update nameservers.
Step Three: If your domain is active, you will see "Manage", menu on the left side. There in the menu list click "Nameservers".
Step Four: Select the option "Use custom nameservers (enter below)"
Step Five: Enter the nameservers, and click "Change Nameserver" button.
NOTE: General issue you might encounter is domain being locked. In that case, go to "Registrar Lock" menu and "Disable Lock", before you can update nameservers.