There are are a few steps first like you said you need a form this form is going to have the fields of the table you want to update. your form should point to a page where it will connect to the database and update the table
page 1 - Form
<form method=post action=update_product.php>
<input type=text name=product_name>
<input type=text name=product_price>
<input type=text name=product_qty></form>
Page 2 The Script
<? // Do some Checks
if (!product_name) {
echo "Required";
}
// connect to DB
$connection_code
$sql = "UPDATE products SET
product_name = '$product_name',
product_price = '$product_price',
product_qty = '$product_qty'";
$result = db_query($sql);
$redirect_to_form;
?>
Hope This helps
GOOD LUCK