You can change the contact information by logging into your account, and then click the My Domains link. You will have a listing of all your domains and in the further most column to the right, click the icon for one of the domains.
Almost at the bottom of the page, is a Registrar Lock check box. You may need to have it unlocked to change the contact information,
At the bottom of the page, click the Contact Information button and you can edit the contact information. After changing the contact info, you should lock the domain so it can't be transferred without your approval.