The following piece of code will update a field of a specific content type by SQL and it is written in a custom module with hook_cron(). Since it updates the database table directly so you have to use it with extreme caution.

function <MODULE>_cron() {
  // Get the list of node ids
  $sql = "SELECT nid FROM {node} WHERE type = :type";
  $result = db_query($sql, array(':type' => '<TYPE>'));
  $nids = array();
  foreach ($result as $row) {
    $nids[] = $row->nid;
  }

  // Update the table directly
  db_update('<FIELD TABLE NAME>')
    ->fields(array('<FIELD NAME>' => '<NEW VALUE>'))
    ->condition('entity_id', $nids)
    ->execute();
  
  // Clear the cache
  drupal_flush_all_caches();
}

Done =)