/**
 * Implements hook_install().
 */
function modulename_install() {
  db_update('system')
    ->fields(array('weight' => 123))
    ->condition('name', 'modulename')
    ->execute();
}

/**
 * Implements hook_install()
 */
function modulename_install() {
  db_query("UPDATE {system} SET weight = 123 WHERE name = 'modulename'");
}

Drupal 7