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();
      
        }