Forgotten it? On the login page, click “Forgot password” and enter your email address. You'll receive a link to set a new password. The link is only valid for a short time.
Want to change it? In the client area under My details → Change password (or via the account menu at the top right).
Your password also applies to SFTP access to your game servers. So after changing it, enter the new password in your SFTP program (e.g. FileZilla).