Your question isn't clear. The heading is 'Usernames' but the question refers to your e-mail address.
You can't change your username without re-registering. (At least, you can't by any automated process. You could try using the 'Contact Us' link, at the bottom of each page, to e-mail a request to the editor).
To change the e-mail address associated with your AB account, go to 'My profile'. In the 'Edit profile' panel, over-write your old address with the new one and click 'Submit'.
Chris