Your domain is registered via SuperHosting.BG
If you have a domain, purchased from SuperHosting.BG, you can change its DNS settings from your customer profile. You can read more useful information in this article: Changing Your Domain DNS Settings
Your domain is not registered via SuperHosting.BG
After you purchase a domain, the company where it was registered should provide you with control panel access or a user account where you can change your settings.
To change the domain DNS, log into your account and look for the DNS Settings or Name Servers menu. After you access the settings, remove the old and enter the new Name servers.
For a website's proper loading there should be at least two name servers entered – Primary Name Server and Secondary Name Server.