The password should be changed only from the Server Manager since many software are involved.
The user can login to the port 980 (Nethgui) or port 9090 (Cockpit - shell access is required) with its own password.
The system will display a simple form to change user information and set a new password.
With this action password for SOGo, Roundcubemail, etc… is changed too.
More info here: http://docs.nethserver.org/en/latest/base_system.html#user-s-profile