Your email address is linked to all of your secure info, so it can't be changed. If you'd like to use a different one, you can close your current account and re-register. To do this, go to your settings page (hover over the little person icon on the top-right corner of this page and click on “Settings”). You’ll see the “Close my account” link in the "Personal information" section.
Afterward, you can sign up again.