How to decrease cookies number in ASP.Net

hi I used below code to save number of selected Item in cookies:

protected void Imgorder_Click(object sender, EventArgs e)
{
    ImageButton ibtn = sender as ImageButton;
    RadioButton RB720 = ibtn.NamingContainer.FindControl("RB720") as RadioButton;
    RadioButton RB1080 = ibtn.NamingContainer.FindControl("RB1080") as RadioButton;
    int id = Convert.ToInt32((sender as ImageButton).CommandArgument);
    DataTable dtFiles = GetFilmInfo(id);
    string ImageName = dtFiles.Rows[0][22].ToString();
    string Code = dtFiles.Rows[0][4].ToString();
    string Daste = dtFiles.Rows[0][1].ToString();
    string P720 = dtFiles.Rows[0][11].ToString();
    string P1080 = dtFiles.Rows[0][13].ToString();
    string F720 = dtFiles.Rows[0][192].ToString();
    string F1080 = dtFiles.Rows[0][193].ToString();
    string Price720 = dtFiles.Rows[0][185].ToString();
    string Price1080 = dtFiles.Rows[0][186].ToString();

    DataTable dt = new DataTable();
    if (Session["Order"] != null)
    {
        dt = Session["Order"] as DataTable;
    }
    else
    {
        dt.Columns.AddRange(new DataColumn[] { new DataColumn("Id",typeof(int)),new DataColumn("ImageName",typeof(string)),new DataColumn("SeP",typeof(decimal)),
                       new DataColumn("code",typeof(string)),new DataColumn("Daste",typeof(string)),new DataColumn("Quality",typeof(string)),new DataColumn("format",typeof(string)),new DataColumn("Se",typeof(string))

             ,new DataColumn("Quantity", typeof(int))});
    }

    if (RB720.Checked || RB1080.Checked || RBT3D.Checked || RBRip.Checked || RBDvd.Checked)
    {
        if (RB720.Checked)
        {
            dt.Rows.Add(dt.Rows.Count + 1, ImageName, Price720, Code, Daste, "720P", "فرمت:  " + F720, " ", "1");
        }
        if (RB1080.Checked)
        {
            dt.Rows.Add(dt.Rows.Count + 1, ImageName, Price1080, Code, Daste, "1080P", "فرمت:  " + F1080, " ", "1");
        }
    }

    int totalOrder = dt.AsEnumerable().Sum(row => row.Field<int>("Quantity"));
    Lblorder.Text = totalOrder.ToString();
    Response.Cookies["UserData"].Value = Lblorder.Text.Trim();
    Session["totalorder"] = Lblorder.Text;
    Session["Order"] = dt;
}

in below code it will save number of row in cookies:

  int totalOrder = dt.AsEnumerable().Sum(row => row.Field<int>("Quantity"));
    Lblorder.Text = totalOrder.ToString();
    Response.Cookies["UserData"].Value = Lblorder.Text.Trim();

in order.aspx page I use gridview to show session's data on it...

and in Lblorder will show cookies["UserData"]

    protected void Page_Load(object sender, EventArgs e)
    {
        if (Request.Cookies["UserName"] != null)
        {
            Lblorder.Text = Request.Cookies["UserData"].Value;

        }
        else {
            Lblorder.Text = "0";
            Lblorder.ToolTip = "سبد خرید شما خالی می باشد.";
        }
        if (!this.IsPostBack)
        {
            BindData();
        }
    }

and:

  private void BindData()
    {
        if (Session["Order"] != null)
        {
            DataTable dt = Session["Order"] as DataTable;
            gvOrders.DataSource = dt;
            gvOrders.DataBind();
            if (dt.Rows.Count > 0)
            {
                decimal totalPrice = dt.AsEnumerable().Sum(row => row.Field<decimal>("SeP"));
                int total = Convert.ToInt32(totalPrice);
                lbltotal.Text = total.ToString("N0") ;
                Session["Total"] = total.ToString();
            }
        }
    }

and in grid view I put remove button to delete data from grid view:

    protected void Remove(object sender, EventArgs e)
    {
        if (Session["Order"] != null)
        {
            DataTable dtOrder = Session["Order"] as DataTable;
            GridViewRow row = (sender as LinkButton).NamingContainer as GridViewRow;
            int id = Convert.ToInt32(gvOrders.DataKeys[row.RowIndex].Value);
            DataRow orderRow = dtOrder.Select().Single(x => Convert.ToInt32(x["Id"]) == (id));
            dtOrder.Rows.Remove(orderRow);
            dtOrder.AcceptChanges();
            Session["Order"] = dtOrder;
            BindData();
        }
    }

in above code it will delete one row from gridview...

now I want when I click on remove button when it delete row from gridview it decrease number of cookies ...

i.e:

value of cookie["userdata"]=4 when I delete row from grid view it will be cookie["UserData"]=3

how I can do it?

Best Regards

Neda