Change email address on iPod Touch

Change it where/how ? If you want to update the primary email address on your account then tap on your id in Settings > iTunes & App Store and log into the account, you should be able to update the primary email address on your account's page. If you want to log out of the account then tap on the id and log out and you can then log in with a different account.

You updated the primary email address on your existing account, you didn't create a new account ?

Tap on your at the bottom of the Featured tab in the App Store app (or in Settings > iTunes & App Store) and you should get a popup with a series of options - tap 'sign out' to log out of the account and then log back in with the updated version of your account so as to 'refresh' the account on it