change @mysql_query(UPDATE `users` (`UID`, `PWD`) VALUES ('{$_POST['UID']}', '{$_POST['PWD']}') WHERE `ID` = "{$_POST['ID']}) or die( 'Query string: ' . $insert . '<br />Produced an error: ' . mysql_error() . '<br />' );
to
mysql_query("UPDATE `users` (`UID`, `PWD`) VALUES ('{$_POST['UID']}', '{$_POST['PWD']}') WHERE `ID` = '{$_POST['ID']}'") or die( 'Query string: ' . $insert . '<br />Produced an error: ' . mysql_error() . '<br />' );