If you have registered your domain name with Darrel Technologies, you should be able to manage the domain name from the Webservice Central Control Panel. Kindly refer to the article on Changing Nameservers of a Domain at the Webservice Central Knowledgebase for information on how to do so.
If you did NOT purchase your domain from us, we are not able to change the name servers because we don't control the domain. Please contact the place where you bought the domain and ask how to make the name server updates. Most domain registrars have a tool so you can do this yourself. Then you will remove the current name servers and replace them with the name servers from your welcome email.