تغییر مقدار یک ستون از mysql table

[sooren_66]

if ( isset($_POST['submit']) )

{

    $link = mysql_connect($mysql_host, $mysql_username, $mysql_password);

    mysql_select_db($mysql_database);

    mysql_query("SET NAMES utf8");

    mysql_query("SET CHARSET utf8");

    $id=$_POST['id'];

$product_id=$_POST['product_id'];

$downloads=$_POST['downloads'];

$query="INSERT INTO tbname(id,product_id,downloads) VALUES ('$id','$product_id','$downloads')"

$result = mysql_query($query);

    echo "ثبت شد";    

}