If you are not logged in and have forgotten your password, simply click on the Forgot Password? link under the Sign In button.
Enter your email address and click the Send Instructions button. You will receive an email containing the next steps to reset your password. If you login with a username instead of a password, selecting the "Forgot password" will take you to an alternate screen where you will be prompted to answer your preselected "challenge" questions so that you can reset your password without requiring email verification.
Please note the sign-in screen will look slightly different for username-enabled communities with the reset password option available within the Sign In Help page.
If you are already logged in and simply wish to change your password, you can do so from the Settings page. If you click the Account menu item, you can change your password using the form below. You will need to know your current password to change it from this form. Click the Save button when you're done.