iTunes 10 for Windows: Change your Apple ID account information
You can change your Apple ID name, password, or billing information at any time.
To change your account information:
Choose Store > View My Account.
Click the Edit button.
Make your changes, and then click Done.
Your changes take effect immediately.
If you also use your Apple ID in other applications, you don’t need to update your information in those applications. The changes you make in iTunes apply to all applications in which you use your Apple ID.
If you have an AOL account, changes you make in iTunes don’t affect your AOL account on the AOL service. For example, if you change your password in iTunes, the password you use to log in to the AOL service doesn’t change.