The good new is your account does offer the ability to replace an email address. Under the Contacts tab of your account you can search or browse for the contact name you are interested in updating. Once you find them, clicking on their name will open to that person's details page. From here, you can click the email address itself or click on the Edit button to the right of the address.
Are you trying to update an unsubscribed contact? If so, these contacts will have to be resubscribed in your account before they can be updated.