How Do I Update Two Rows In Datagridview?

Violation of Primary key

string strPRSconn = ConfigurationManager.ConnectionStrings["POSdb"].ConnectionString;
            string date = dt_date.Value.ToString("yyyy-MM-dd");
            var counter = 1;
                SqlConnection sc = new SqlConnection(strPRSconn);
                sc.Open();
                counter++;
                foreach (DataGridViewRow row in dataGridView1.Rows)
                {
                    
                if (!row.IsNewRow)
                {
                    
                        string strQry = "UPDATE CounterChecking "+
                        "SET part_no = @c0,po_no = @po_no,invoice = @invoice,part_name = @c1,"+
                        "date = @date,color = @c2,qty_ordered = @c3,qty_for_delivery = @c4,qty_recieved = @c5,"+
                        "variance = @c6,unit_price = @c7, total = @c8, remarks = @c9 "+
                        "WHERE po_no= '" + cb_po_search.Text + "'";
                        SqlCommand scmd = new SqlCommand(strQry, sc);
                        scmd.Parameters.AddWithValue("@c0", counter);
                        counter++;
                        scmd.Parameters.Add("@po_no", SqlDbType.VarChar).Value = tb_ponumber2.Text;
                        scmd.Parameters.Add("@invoice", SqlDbType.VarChar).Value = tb_invoice2.Text;
                        scmd.Parameters.AddWithValue("@c1", row.Cells[1].Value);
                        scmd.Parameters.Add("@date", SqlDbType.Date).Value = date;
                        scmd.Parameters.AddWithValue("@c2", row.Cells[2].Value);
                        scmd.Parameters.AddWithValue("@c3", row.Cells[3].Value);
                        scmd.Parameters.AddWithValue("@c4", row.Cells[4].Value);
                        scmd.Parameters.AddWithValue("@c5", row.Cells[5].Value);
                        scmd.Parameters.AddWithValue("@c6", row.Cells[6].Value);
                        scmd.Parameters.AddWithValue("@c7", row.Cells[7].Value);
                        scmd.Parameters.AddWithValue("@c8", row.Cells[8].Value);
                        scmd.Parameters.AddWithValue("@c9", row.Cells[9].Value);
                        scmd.ExecuteNonQuery();
                        sc.Close();
                        MessageBox.Show("Successfully Updated!");}
                sc.Close();
            }
        }