Update help

[rollerboy]

Hi I have a shopping cart that displays the products that a customer buys. When a customer buys a product only one is added to the cart. I would like a way of updateing the quantity in the cart. The code for the cart is below. I would be grateful for any suggestions?
ps I'm new to php.

<?php
include ("Products_inc.php");

//call to stylesheet on include file
HeaderAndStylesheet();

$query= "SELECT*FROM cart WHERE Username='$Username'";
$result=mysql_query($query);
$num=mysql_num_rows($result);

if ($num ==0)
{
echo "The database contains no contacts yet";
}
else
{
echo "<b><centre> Cart</centre></b><br><br>";
//echo "$Full_name";
?>

<table border ="1" cell spaceing= "2" cellpadding= "2">
<tr>
<th><font face= "Arial, Helvetica, sans-serif"> order id</font></th>
<th><font face= "Arial, Helvetica, sans-serif"> Product Name</font></th>
<th><font face= "Arial, Helvetica, sans-serif"> Username</font></th>
<th><font face= "Arial, Helvetica, sans-serif"> Product Description</font></th>
<th><font face= "Arial, Helvetica, sans-serif"> Price</font></th>
<th><font face= "Arial, Helvetica, sans-serif"> Quantity</font></th>
<th><font face= "Arial, Helvetica, sans-serif"> Delete</font></th>
</tr>
<?
$i=0;
while ($i<$num)
{
$order_id=mysql_result($result, $i, "order_id");
$Name=mysql_result($result, $i, "Name");
$Username=mysql_result($result, $i, "Username");
$Description=mysql_result($result, $i, "Description");
$Price=mysql_result($result, $i, "Price");
$Quantity=mysql_result($result, $i, "Quantity");
?>
<tr>
<td width=100><font face="Arial,Helvetica, sans-serif"><? echo $order_id?></font></td>
<td width=100><font face="Arial,Helvetica, sans-serif"><? echo $Name?></font></td>
<td width=100><font face="Arial,Helvetica, sans-serif"><? echo $Username?></font></td>
<td width=150><font face="Arial,Helvetica, sans-serif"><? echo $Description?></font></td>
<td width=100><font face="Arial,Helvetica, sans-serif"><? echo "£$Price"?></font></td>
<td><font face="Arial,Helvetica, sans-serif"><? echo $Quantity?></font></td>

<td><font face="Arial,Helvetica, sans-serif">
<?
echo "<form action=\"delete_from_cart.php\" method=\"post\">";
echo "<input type=\"submit\" value=\"Delete\">";
echo "<input type=\"hidden\" name =\"order_id\" value=$order_id>";
echo "<input type=\"hidden\" name =\"Username\" value=$Username>";
echo "</form>";
?>
</td>
</tr>
<?
$i++;
}
echo "</table>";
}
echo "<form action=\"display_product.php\" method=\"post\">";
echo "<input type=\"submit\" value=\"Continue shopping\">";
echo "<input type=\"hidden\" name =\"Username\" value=$Username>";
echo "<input type=\"hidden\" name =\"Full_name\" value=$Full_name>";
echo "</form><br>";

echo "<form action=\"check_out.php\" method=\"post\">";
echo "<input type=\"submit\" value=\"Go to check out\">";
echo "<input type=\"hidden\" name =\"Username\" value=$Username>";
echo "<input type=\"hidden\" name =\"Full_name\" value=$Full_name>";
echo "</form>";
?>