Using Entity Wrapper to easily save yo entities.

gistfile1.txt

function _update_multi_registration($flag, $entity_id, $status) {
  $entity_type = $flag->entity_type;
  $entity = entity_load_single($entity_type, $entity_id);
  $nodewrapper = entity_metadata_wrapper($entity_type, $entity);
  $nodewrapper->field_event_mult_reg = $status;
  $nodewrapper->save(true);
}