If you are not logged in and have forgotten your password, simply click on Forgot Password? next to the Sign In button.
You will receive an email containing the next steps to reset your password.
If you are already logged in and merely wish to change your password, you can do so from the Settings page. If you click the Account tab, then the Edit link in the upper right hand corner of the Account box, you can change your password using the form below. Click the Save button when you're done.