To change your email here you need to still have access to the old email. Go into your User CP/Edit Your Details/Edit Email and Password to make the change. An email with a confirmation link will be sent to the old email address. When you click on that the change will be confirmed. Until you do that you will not be able to post anywhere but the Welcome Board. If you have any problems or don't get the email you can check out the thread stickied at the top of the Welcome Board for help.
There is not a way to change a username. You would need to start a new account. You can leave the email on this account as it is and just use the new email to start over if you would rather have a new name.