Update values in DataBase by adding TextBox Value in ASP.Net

How do I add value in textboxt asp.net, when I click ADD, then in the field with the ID 111 so QTY 4,

My code error: I'm sure my code is wrong.

Using con As New SqlConnection(connStr)
con.Open()
CMD = New SqlCommand("SELECT * FROM tbl_produk WHERE kode='" + kode.Value + "'", con)
RD = CMD.ExecuteReader
RD.Read()
If RD.HasRows Then
   If Quantity.Text > RD.Item(8) Then
        Response.Write("<SCRIPT>alert('Stok Barang Terbatas Hanya Ada " + stok.Text + " Silakan Masukan Di Bawah Stok Saja')</SCRIPT>")
   Else
       CMD = New SqlCommand("SELECT * FROM tbl_DetailCart WHERE id='" + id + "' AND kode='" + kode.Value + "'", con)
       RD = CMD.ExecuteReader
       RD.Read()
       If RD.HasRows Then
         'Dim c As Integer = RD.Item("jumlah") + Convert.ToInt32(Quantity.Text.Trim)
          Dim update As String = "UPDATE tbl_DetailCart SET jumlah='" + RD.Item("jumlah") + "' + " + Convert.ToInt32(Quantity.Text.Trim) + " WHERE id='" + kode+ "' AND kode='" + kode.Value + "'"
          CMD = New SqlCommand(update, con)
          CMD.ExecuteNonQuery()
              Response.Redirect("~/carting.aspx")
          Else
              Dim simpan As String = "INSERT INTO tbl_DetailCart([id],[kode],[produk_nama],[produk_harga],[jumlah],[total]) VALUES " &
              CMD = New SqlCommand(simpan, con)
              CMD.ExecuteNonQuery()
              Response.Redirect("~/carting.aspx")
       End If
   End If
End If
con.Close()
End Using