To update your nameservers at CirtexDomains.com:

1. Go to http://www.cirtexdomains.com
2. Log in (near top right corner of page) with your login ID and password
3. Click Registered Domains in the QuickLinks tab
4. Click on the domain name you'd like to edit from the available list
5. Scroll down and click on DNS Settings
6. Click Edit, to the right of DNS Servers
7. Click the dot placed in front of Custom DNS, which opens a place to add nameservers
8. In the '1' slot type ns1.cirtexhosting.com, or your current nameserver
9. In the '2' slot type ns2.cirtexhosting.com, or your second nameserver
10. Click Save Changes (button near the bottom of the page)
    
    You will see a message stating "Any change in DNS Servers performed in this section will take approximately 24 hours to propagate.)
11. Click OK 

See also All About Nameservers, Set Up Custom Nameservers