$node = node_load(17531);
// for each field whose value remains unchanged
unset($node->field_status_dvig_po_sklady);
// for each field whose value changes
$node->field_status_dvig_po_sklady['und'][0]['value'] = 's0';
field_attach_update('node', $node);
entity_get_controller('node')->resetCache(array($node->nid));
Ещё здесь https://drupal.stackexchange.com/questions/21023/updating-a-field-programmatically-hook-node-update
