how yo update session

hi

I heve some textbox and button in Info.aspx when I click on button it will save data into session and redirect to order.aspx...

   protected void LBsabt_Click(object sender, EventArgs e)
    {
        UserData user = new UserData
        {
            Name = Txtname.Text,
            State = DdlState.SelectedItem.Text,
            City = DdlCity.SelectedItem.Text,
            Mobile = TxtMob.Text,
            Tell = txttell.Text,
            Address = Txtaddu.Text,
            PostCode = TxtcodeU.Text,
            Email = TxtEmail.Text,
    };
        Session["UserData"] = user;
        if (Session["UserData"] != null)
        {
            UserData userData = Session["UserData"] as UserData;
            string Name = userData.Name;
            string city = userData.City;
            string state = userData.State;
            string mobile = userData.Mobile;
            string tell = userData.Tell;
            string address = userData.Address;
            string postCode = userData.PostCode;
            string email = userData.Email;
        }
 Response.Redirect("order.aspx");
        }

in order.aspx is back button that will back to Info.aspx page in info.aspx I put below code that will show session value in textboxes

      if (Session["UserData"] != null)
        {
            UserData userData = Session["UserData"] as UserData;
            if (userData != null)
            {
                Txtname.Text = userData.Name;
                TxtMob.Text = userData.Mobile;
                txttell.Text = userData.Tell;
                TxtcodeU.Text = userData.PostCode;
                Txtaddu.Text = userData.Address;
                TxtEmail.Text = userData.Email;

            }
        }

now I want when I back to info.aspx page and change text into textboxs it will change sessions value...

i.e in Txtname is "Neda" that saved before in session now I want change it to "neda shahbazi" when I change textbox text and click on button it doesn't change usedata.Name I mean it is "Neda" same az before that saved in session

I want when I change textboxs text it will update session that saved before...

Best regards

neda