Need Help Updating A Quantity In Db Via A Loop

When a user orders an amount of a given product, I want to update the available quantity in my db.  I use a loop to insert each ordered product item and the needed quantity into an ORDERED_ITEMS table (these items are tied to an order record in another table).  The available_quantity is in the STOCK table.  Here is my code.  Can you help me out with how I can achieve this? (I know my queries should be in SP's, but for now while I'm getting everything the way I want it, it's easier to write it directly in my code.)

 Thanks!

 //insert the individual items
string items_to_ship = "";

foreach (GridViewRow rw in GridView1.Rows)
{
if (rw.RowType == DataControlRowType.DataRow)
{
if (((TextBox)rw.Cells[4].FindControl("qtyneeded")).Text != "")
{
int qty = Convert.ToInt32(((TextBox)rw.Cells[4].FindControl("qtyneeded")).Text);
int itm = Convert.ToInt32((rw.Cells[0].Text));

SqlConnection mycn2 = new SqlConnection(myConnectionStr);
SqlCommand myCommand4;
SqlDataReader rdr = null;
int new_qty = 0;
myCommand4 = new SqlCommand
("INSERT INTO SHP_ORDERED_ITEMS (ship_id, item_id, quantity) values(@ship_id,@item_id,@quantity);" +
"SELECT item_description, qty_available from shp_stock where item_id = @item_id;", mycn2);

myCommand4.Parameters.Add("@ship_id", SqlDbType.UniqueIdentifier).Value = shipmentID;
myCommand4.Parameters.Add("@item_id", SqlDbType.Int).Value = itm;
myCommand4.Parameters.Add("@quantity", SqlDbType.Int).Value = qty;

myCommand4.Connection.Open();
rdr = myCommand4.ExecuteReader();
while (rdr.Read())
{
// create a list of item descriptions and quantities of each, for use in the email
items_to_ship += (string)rdr["item_description"] + " (" + qty.ToString() + ")<br>";


}
myCommand4.Connection.Close();

}
}
}