If you want to purchase and download items from the iTunes Store, iTunes requires that you create an account. It uses the email given during registration as your default Apple ID for logging into iTunes services and for sending receipts for purchases. If your primary e-mail address changes, you can edit your account information to use a new default e-mail address.
Launch iTunes, and then select "Sign In" from the "Store" toolbar menu. Type your current account credentials and click "Sign In."
Select "View My Account" from the "Store" menu, type your account credentials again and then click "View Account." Click the "Edit" button beside your Apple ID on the "Account Information" page.
Type the new e-mail you want to use beside "Email," and then click "Done" to change the default e-mail address associated with your iTunes account.