This depends on the registrar service you originally used to register your domain name. If you arent sure who you registered your domain with, you may perform a WHOIS search here: NanoServers WHOIS
Simply type in the domain name, select the appropriate Top Level Domain extension (.com, .net, .org, etc.) and click the "Go" button. The last of the three sections - "Contact Type Technical" - should contain the necessary information as to who your registrar is. In case it does not, you may follow a similar procedure at Network Solutions which should return a definitive result in case your domain is registered with certain smaller or national registrars.
If you registered your domain with NanoServers Registrar, log on to myCP page and click the Manage Domains link on the right in the "Domain Names" section. From there click the link for the specific domain name you would like to manage, and the various options presented on the next page will allow you to change these options.
Some NanoServers customers may need to open a support ticket via their myCP for DNS changes or the addition of certain types of DNS records. Additionally, those who are not wholly comfortable with the above process may also open a support ticket to request a change. Please note that we cannot make changes via phone, and some types of changes may incur support credit charges.
If your domain is not registered with NanoServers, you will need to use the contact information provided by WHOIS to contact your registrar. Most registrars will provide a search feature for their support database where you can find specifics on managing DNS with their service by searching for phrases like "Manage DNS" or "change DNS".
The following links are to relevant FAQs, how-tos, and tutorials for managing DNS on a few of the major registrars.