Generally you do this in your domain registration panel with the company that you registered your domain with.
If your DNS is self hosted, you will have to update the DNS on your own server (I don't think this is you).
Godaddy, MyDomain etc all have panels to update DNS records.
Could you specify where you registered your domain ?
You can change which service your domain name uses for its website and email by managing its DNS (Domain Name System) settings. You don't need to know many of the details to use this article, but you can find more information in What is DNS?
Using our DNS Manager, you can change your domain name's DNS settings (stored in something called a zone file). You can use our DNS Manager if:
Your domain name is registered through us and uses our nameservers
Your domain name is registered somewhere else and you set up Off-Site DNS
If your domain name isn't in either of these situations, you can contact the company who controls your domain's nameservers to control its DNS/zone file settings.
Premium DNS Users: Instead of these instructions, see Managing Premium DNS for Your Domain Names.