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.)  Enter the new password in the blank box next to the email address for which you want to change the password.  Then, click on the "save" link next to the new password you just entered.