i am new in asp.net. i have a problem in gridview.

protected void Grid1_RowUpdating(object sender, GridViewUpdateEventArgs e)
       {
           int i = e.RowIndex;
           int pid = Convert.ToInt32(Grid1.DataKeys[e.RowIndex].Values[0].ToString());
           string uid = Grid1.DataKeys[e.RowIndex].Values[1].ToString();
           Label lbl2 = (Label)Grid1.Rows[i].Cells[5].FindControl("lbl1");
           TextBox txtlbl1 = (TextBox)Grid1.Rows[i].Cells[4].FindControl("txtlbl");
           StringBuilder str = new StringBuilder();
           for (int j = 0; j < Grid1.Rows.Count; j++)
           {
               GridViewRow row = Grid1.Rows[i];
               //CheckBox aa = (row.FindControl("checkbox1") as CheckBox).Checked;
               bool isChecked = ((CheckBox)row.FindControl("checkbox1")).Checked;
               if (isChecked)
               {
                   str.Append(Grid1.Rows[i].Cells[3].Text);
               }

           }
           string str1 = txtlbl1.Text;
           SqlCommand cmd = new SqlCommand("update Product_order set Extend_date='" + txtlbl1.Text + "'where P_id=" + pid , con);
           cmd.ExecuteNonQuery();
           Grid1.EditIndex = -1;
           buildgrid();
           string BID = auto_ID();
           Session["back_office"] = BID;
           cmd = new SqlCommand("insert into Backofc_master (B_ID) values ('" + BID + "') ", con);
           cmd.ExecuteNonQuery();

       }