You can change the default nameserver of your server from your WHM Root Account.
Note: Root Access is required. You cannot access it from the WHM Reseller Account.
1. Login to your WHM Root Account.
2. Go to Server Configuration >> Basic WebHost Manager® Setup or type Basic WebHost in the search box, and the option will appear. Click on it.
3. Scroll to the end of the page, Under Nameserver, enter your Nameserver in the text field.
4. Click on the Save Changes button.
The nameserver will be changed successfully.