how do I change my account password

You can log into your account on your computer's iTunes via the Store > View My Apple Id menu option at the top of iTunes (if you are on iTunes 11 on a PC then Alt-S should get the Store menu to appear), and when you've logged in click the Edit button to the right of your id towards the top of the screen that you are taken to.

Or you can do it via this page : https://appleid.apple.com/