@abc.abstractmethod<EOL><INDENT>def update_value(self, value_form):<DEDENT>
pass<EOL>
Updates an existing value.

        :param value_form: the form containing the elemnts to be updated
        :type value_form: ``osid.configuration.ValueForm``
        :raise: ``IllegalState`` -- ``value_form`` already used in an update transaction
        :raise: ``InvalidArgument`` -- the form contains an invalid value
        :raise: ``NullArgument`` -- ``parameter_id, value_id`` or ``value_form`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure
        :raise: ``Unsupported`` -- ``value_form`` did not originate from ``get_value_form_for_update()``

        *compliance: mandatory -- This method must be implemented.*