Simply go to your domain registers web site, and look around for links, which point to something like, domain manager, manage domain, or something of that administrative nature. 

In your welcoming email, you were sent DNS settings, which look similar to this example:

NS1.YOURDOMAIN.COM (216.127.78.7)
NS2.YOURDOMAIN.COM (216.127.78.156)

Most of the newer registers such as the (OPEN SRS) based entities have turned this into a 5-minute process. 

You simply login to the register, select 'manage domain' and you'll be presented with an option to update your new DNS numbers. 

Contrary to popular belief, Network Solutions 'now' also provides an online interface to change these settings, so this process with them is no longer as complicated as it use to be, however it's still not as simple as the OPEN SRS based systems.  

If your particular register 'does not' provide a domain manager of some type, then you'll need to send them a message requesting a change of DNS. 

Once you've accessed the "management interface" of your domain name, look for a setting, which says "change or manage DNS settings." 

In most cases, you can simply cut and paste the DNS settings we've sent you directly into the spaces, which correspond to your DNS management settings.