I Pad 2 Apple ID/ password change creating problems

[Aardvark13]

Help please. Having recently change my service provider, I have set up a new email account and want everything on my i Pad to use the new one by default. I believe I have successfully changed my Apple ID and password and they appear to work. However, i Tunes and other related it peripheral facilities still show my old email address and refuse to accept any passwords! I had expected that changes made to the main Apple ID and password would automatically work for other areas requiring Apple ID,and password. This seems not to be the case.

How can I change the ID and password on i Tune and other facilities? I can find no obvious means of doing this.

Any help will be greatly appreciated.