If you are already logged in and simply want to change your profile password, click Account Settings in your profile and change the password in Settings.
If it's asking for your old password, and you don't remember it, completely log out first. Then, at the log-in screen, follow the "Forgot Password" link at the bottom and follow the prompts.
If you are still having trouble, be sure to clear your browser's cache and cookies, or choose a new browser or device, then try these steps again.