Q: I have a credit on my I tunes account and have made several purchases but now cannot find the button to allow the change agains my existing credit. Can anyone help

Hello there bex1601,

This article will show how to have no credit card assigned to your Apple ID:

iTunes Store: Changing account information

http://support.apple.com/kb/ht1918

Changing your account information using a computer:

1. Open iTunes.
2. Choose Store > Sign In.
3. Enter your Apple ID and password, then click the Sign In button.
4. Choose Store > View My Account.
5. Enter your account password, and click the View Account button.

Then:

To update your name, billing address, or payment information, follow these instructions:

1. From the Apple Account Information page, click the Edit Payment Information button.
2. Edit the information that you would like to change.Note: The payment methods that the iTunes Store accepts can be found in the payment type section. If you do not want a payment method on your account, select None in the payment type section.

Also here is this bit of info about the order in which credits vs credit card are charged for purchases:

If you have no free song credits or gift certificates, prepaid cards, or allowance credits, your credit card is billed for the songs you buy.

From: iTunes Store: How iTunes Store purchases are billed

          http://support.apple.com/kb/HT5582

Regards,

Sterling