The email address you provide during registration becomes your username for the purposes of logging into the applicaiton. You can update this email address using the Update Email Address button in the Settings window. Your will prompted to enter the new email address twice to ensure accuracy. When complete, click Save to save the new email address (username) which will become the email address used login.
You will need to use the new email address the next time that you logout and login to the application. As long as the current application session is active, your previous email address will be active.