include("bd.php");
if (isset($_POST['Mark']) && !empty($_POST['Mark'])) {
$result = mysql_query("UPDATE `marktable` SET `Mark` = '" . $_POST['Mark'] . "' WHERE `id` = 13");
if ($result) {
echo 'ok';
} else {
echo 'no';
}
}
<form action="" method="POST">
<input type="text" name="Mark">
<input type="submit" value="Сохранить">
</form>