Data Controls :: How To Change Session Data After Selecting Item From Dropdownlist

According below code I save data in session:

protected void Imgorder_Click(object sender, EventArgs e)
{
ImageButton ibtn = sender as ImageButton;
int id = Convert.ToInt32(Request.QueryString["Id"].ToString());
DataTable dtFiles = GetFilmInfo(id);
string Name = dtFiles.Rows[0][1].ToString();

[Code] ....

In gridview I define dropdownlist that when change I tem it will change price value...

protected void DdlQuantityS(object sender, EventArgs e)
{
DropDownList ddlQuantity = (sender as DropDownList);
Label quantity = ddlQuantity.NamingContainer.FindControl("LblQuanyity") as Label;
quantity.Text = ddlQuantity.SelectedItem.Text.Trim() == "Select" ? "0" : ddlQuantity.SelectedItem.Text.Trim();
Label price = ddlQuantity.NamingContainer.FindControl("LblPrice") as Label;

[Code] ....

And according below code it will show quantity number in dropdownlist:

protected void OnRowDataBound(object sender, GridViewRowEventArgs e)
{
using (SqlConnection conn = General.GetConnection())
{
using (SqlCommand cmd = General.GetCommand("OrderNum", conn))

[Code]....

Now I want when I change dropdownlist Item from gridview it will save quantity(selected Item from dropdownlist) and price and priceT in seesion["Order"] that created in ImgorderM_Click metod...

How I can do it?