To update the email address you login with, follow these easy steps.
1. Login with your existing email.
2. Click on "Your Profile" from the Account dropdown in the right-hand corner of your account.
3. From your profile, delete the existing email address and enter in the new one. Scroll down the page to enter in your existing password, and click Save in the upper right hand corner of the screen.
From now on, you will login with the new email you just entered.