You can change your password - if you are logged in - via the 'My profile' page. On the profile page you will find the 'Change password' option.
The new password applies to all websites of Coret Genealogie (Family Archivist, Genealogy Online, Open Archives, Stamboom Forum and the Stamboom Gids).
Make sure you set a password that is difficult to guess that you only use for the Coret Genealogy websites. The password must consist of at least 9 characters (more is better).