Can't Insert & update to database Error SQLException

Question

Can't Insert & update to database This Error

Violation of PRIMARY KEY constraint 'PK_Product'. Cannot insert duplicate key in object 'dbo.Product'.
The statement has been terminated.


Error Line -->   cmd.ExecuteNonQuery();

 String Con = "Data Source=.\\SQLEXPRESS;AttachDbFilename=|DataDirectory|"
                     + "\\TCTSuperMarket.mdf;Integrated Security=True;User Instance=True";//???????????? string ????????????????????????????
        SqlCommand cmd = new SqlCommand();

 private void btedit_Click(object sender, EventArgs e)
        {
            btsave.Enabled = false;
            string strSQL;
            SqlConnection Conn = new SqlConnection(Con);
            //try
            //{
                strSQL = "UPDATE Product SET " +
              " ProID = '" + this.txtProID.Text + "' " +
              " ,ProName = '" + this.txtproname.Text + "' " +
              " ,Detail = '" + this.txtdetailpro.Text + "' " +
              " ,NumInventory = '" + this.txtnuminventory.Text + "' " +
              " ,cost = '" + this.txtcost.Text + "' " +
              " ,Price = '" + this.txtprice.Text + "' ";
                cmd = new SqlCommand();
                cmd.Connection = Conn;
                cmd.CommandText = strSQL;
                cmd.CommandType = CommandType.Text;
                Conn.Open();
                cmd.ExecuteNonQuery();
                MessageBox.Show("?????????????????????????????! ", "??????", MessageBoxButtons.OK, MessageBoxIcon.Information);

            //}
            //catch (Exception ex)
            //{
            //    MessageBox.Show("??????????????????????? ! ?????????????????????????????????  ", "?????????!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
            //}


            

        }

 private void btsave_Click(object sender, EventArgs e)
        {
            int intnumrows;
            SqlConnection Conn = new SqlConnection(Con);
            Conn.Open();
            String strSQL;
            strSQL = "SELECT COUNT(*) FROM Product WHERE ProID = '" + this.txtProID.Text + "' ";
            cmd = new SqlCommand(strSQL, Conn);
            intnumrows = Convert.ToInt32(cmd.ExecuteScalar());
            if (intnumrows > 0)
            {
                MessageBox.Show("?????????????????????????????????????????? ????????????!", "?????????!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
                txtProID.Focus();
            }
            else
            {
                if (txtProID.Text == "" || txtproname.Text == "" || txtdetailpro.Text == "" ||
                    txtnuminventory.Text == "" || txtcost.Text == "" || txtprice.Text == "")
                {
                    MessageBox.Show("?????????????????????????", "???????", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
                    txtProID.Focus();
                }
                else
                {
                    strSQL = "INSERT INTO Product(ProID,ProName,Detail,NumInventory,Cost,Price)" +
                           " values(@ProID,@ProName,@Detail,@NumInventory,@Cost,@Price)";
                    // SqlCommand cmd = new SqlCommand("INSERT INTO Product(ProID,ProName,Detail,NumInventory,Cost,Price)" +
                    //        " values(@ProID,@ProName,@Detail,@NumInventory,@Cost,@Price)", Conn);
                    cmd = new SqlCommand(strSQL, Conn);
                    cmd.Parameters.AddWithValue("@ProID", txtProID.Text);
                    cmd.Parameters.AddWithValue("@ProName", txtproname.Text);
                    cmd.Parameters.AddWithValue("@Detail", txtdetailpro.Text);
                    cmd.Parameters.AddWithValue("@NumInventory", txtnuminventory.Text);
                    cmd.Parameters.AddWithValue("@Cost", txtcost.Text);
                    cmd.Parameters.AddWithValue("@Price", txtprice.Text);
                    try
                    {
                        Conn.Open();
                        cmd.ExecuteNonQuery();
                        ShowData();
                        MessageBox.Show("??????????????????????????", "?????????????????????", MessageBoxButtons.OK, MessageBoxIcon.Information);
                        txtProID.Text = "";
                        txtproname.Text = "";
                        txtdetailpro.Text = "";
                        txtnuminventory.Text = "";
                        txtcost.Text = "";
                        txtprice.Text = "";
                        Conn.Close();
                    }
                    catch (Exception)
                    {
                        MessageBox.Show("Error!! ?????????????????????????", "?????????", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
                    }

                }
            }
        }