Note that downloaded content (apps, some music) are linked to the AppleID used to purchase them, so changing your AppleID can cause difficulties with that purchased content. If you only want to change your +email address+ you can keep the same AppleID (even though you're no longer using that email account). To change your email contact information, _do not_ change in iTunes on your computer. Instead, go to the site below and click on email to edit that information.
Should you want to change your AppleID anyway, you can also do that using the link below, or via iTunes on your computer.
http://myinfo.apple.com