Have you configured your account so that you are part of the "Customer Portals Administrators" web role? Once you do that and log into the portal, you will have full access to the CMS capabilities of the portal. To change the portal
label, simply hover over the text and you will see an edit button pop up. Simply click the edit button and change the content. When you click the save button on the rich-text editor it will update the content in CRM.
I would highly recommend reading all of the documentation included in the portal. One document that covers this topic is
Customer Portal Content Management Guide_V1.0.doc