The International WHOIS Database is a searchable index that displays the contact information for the owners of every domain. Updating the Contact Info for your domain alters the info shown in this WHOIS Database. These changes can be made via the Domains tab of your account.
Go to the Domains tab and click the Manage button for the domain you'd like to update.
The WHOIS Contact Info can be seen on the right side of the screen. Click Edit to change this info.
Make any changes you'd like to make and hit Save.
If you want to get overly fancy, you can select the Advanced Contact Settings option and set up various contacts for your domain name: A Registrant Contact, an Admin Contact and a Technical Contact. But this isn't in any way necessary; generally speaking the same contact information is used for each of the three.
Your updated information will show up in your account with us immediately. But it may take up to 24 hours or so for the change to reflect in the WHOIS Database.