On your iPad:
Go to Settings - iTunes & App Stores, tap Apple ID, then choose "View Apple ID", enter password.
Now tap Apple ID again. You'll see the information about your apple ID there.
First, there's Apple ID. Then you'll see "Email". This is where your emails will be sent to. If it isn't existing any more, you should change this. You can do it right there. Then tap "Done" (upper right corner). If you don't have a rescue email address, you won't see "Email".
Third position is "Password", then "Verify". I havent't tried that yet, but perhaps you should be able to change your password there as well. Don't forget to tap "Done" when you've finished.