Desde ya gracias...
Código PHP:
$cerro=$_POST['cerro'];
mysql_select_db($database_connection, $connection);
$ultimo=mysql_query("SELECT MAX(idregistro)ultimo FROM registro ");
$ultimo = mysql_fetch_assoc($ultimo);
$ultimo['ultimo'];
mysql_select_db($database_connection, $connection);
$update=mysql_query("UPDATE registro SET cerro='$cerro' where idregistro='$ultimo'");