From your Billing page, to to the Domains dropdown (at the top) and choose Domains. Once on that page you can make the adjustments you need.
FYI, by making any changes here, your domain name is subject to a 60-day Change of Registrant lock. You cannot transfer a domain name to a different registrar within 60 days of making changes to the registrant name, organization or email address (or the Administrative Contact email address, if there is no registrant email address).
