Note: This only applies to SiteDart Hosting services. Each registrar may have differences...
You can change the nameservers of your registered domain from control panel itself. For that, you will need to log https://store.sitedart.net/clientarea.php
Once you logged in, click on Domains -> My Domains
After that, click on Manage Button icon for your domain name for which you want to change the nameservers (or click the 'Active' button).
Click on the Nameservers tab, apply the nameservers which you want to set for your domain.
Click on Change Nameservers to apply the changes.