If you know your password and would like to change it, you can do so in the Password section on your Account Settings page, which you can access from the Account drop-down menu at the top of any page.
Unfortunately, if you have forgotten your password, you will first have to log out then request a password reset. Before you do so, be sure that the contact email listed in your Account Settings page is one that you currently have access to, since the password reset email will be sent to this address. You can request a password reset by clicking the “Forgot your password?” link located near the “Login” button. [source]