Depending upon the domain name registrar where you registered the domain name, you should have access to the registrar control panel. You can login to your domain name registrars control panel and make the required nameserver updates.
Please use the following instructions for popular domain name registrars. Note: if your registrar is not listed below, please contact them to obtain the instructions for updating your domains name servers.
Godaddy.Com
http://help.godaddy.com/article/664
NetworkSolutions.Com
http://www.networksolutions.com/support/move-dns-to-a-new-server/
Enom.Com
http://www.ehow.com/how_2044662_change-nameservers-enom-account.html
Register.Com
http://help.register.com/cgi-bin/register_help.cfg/php/enduser/std_adp.php?p_faqid=1252
Netfirms.Com
1. Login to your Netfirms Control Panel.
2. Click Domains.
3. Click Domain Manager.
4. Click on the “+” button to the left of the domain name.
5. Click the DNS tab.
6. Click the DNS Server sub-tab.
7. Update the name server information in the proper field.
8. Click Modify.