Log into your account, click on the "Settings" tab in the upper right hand corner.  Then click on the "Manage Lines" link in the upper right hand corner.  (If you do not see a "Manage Lines" link they you do not have a corporate account.  To find out more about corporate accounts, click here.)  Replace the email address for the account that you want to change and click the "save" on the same line as where you made the change.