You can change your billing address, switch the credit card you have on file for Store purchases, or edit other information in your Apple Account without calling Apple. Just launch iTunes, click the Store icon in the Source list, and then sign in to your account by clicking the Sign In button in the upper-right corner of the screen.
Once you've signed in, you'll see your account name (email address). Click it. In the box that pops up, re-enter your password and click View Account. If you want to change your password or secret identity-proving question, click the Edit Account Info button. To change your billing address or credit card information, click the Edit Payment Information button. You can also deauthorize all the computers that can play songs purchased with this account (more on when and why you'd want to do that later).
And there are other account settings you can change here, too, like the nickname that appears when you post a customer review in the Store. You can also turn on the My iTunes feature, which lets you put widgets and news feeds on your blog or Facebook profile showing all the things you've been snapping up in the Store lately.
By the way, any changes you make to your Apple Account through iTunes affect other programs ...