This example will always show the success message when an entry is updated, even if the setting tells the form to redirect.

add_filter('frm_success_filter', 'change_my_confirmation_method', 10, 2);

function change_my_confirmation_method( $type, $form ) {

  if ( $form->id == 5 && isset( $_POST ) && isset( $_POST['frm_action'] ) && $_POST['frm_action'] == 'update' ) { //change 5 to the ID of your form

    $type = 'message';

  }

  return $type;

}

The example below will force a success message and set the message to "The form has been received."

add_filter('frm_success_filter', 'force_change_success_message', 10, 2);

function force_change_success_message( $type, $form ) {

  if ( $form->id == 5 && isset( $_POST ) && isset( $_POST['frm_action'] )) { //change 5 to the ID of your form

    $type = 'message';

	$form->options['success_msg'] = "The form has been received.";

  }

  return $type;

}

Use this code example to check if there is any redirect action. If there is any, remove the redirect action when updating an entry.

add_filter('frm_success_filter', 'prevent_redirect_action' , 10, 2);

function prevent_redirect_action( $type, $form, $action ) {

	if ( $form->id != 5 || 'update' != $action ) {

		return $type;

	}
	if ( 'redirect' == $type ) {

		return 'message';

	}
	if ( is_array( $type ) ) {

		foreach ( $type as $index => $conf_action ) {

			if ( 'redirect' == FrmOnSubmitHelper::get_action_type( $conf_action ) ) {

				unset( $type[ $index ] );

			}

		}

	}

		return $type;

}