How Library return system works? C#.net and MySQL WB

//and here you can put a value to subtract the value of stock from database.TableA 
and the value u use will be inserting directly to stock.database.tableB

private void brwBtn_Click(object sender, EventArgs e)
        {
            try
            {
                int brValue = 0;
                brValue = int.Parse(stockTxt.Text) - int.Parse(brStockTxt.Text);

                string modConnection = "datasource=localhost;port=3306;username=root;password=1234";
                string modQuery = "UPDATE Database.tableA SET Id = '" + this.idTxt.Text + "',Items = '" + this.itemsTxt.Text + "',Stock = '" + brValue + "' Where Id = '" +this.idTxt.Text + "';";
                    
                MySqlConnection modData = new MySqlConnection(modConnection);
                MySqlCommand modCommandData = new MySqlCommand(modQuery, modData);
                MySqlDataReader MyReader;

                modData.Open();
                MyReader = modCommandData.ExecuteReader();
                MessageBox.Show("Registered Data..");

                this.Hide();
                while (MyReader.Read())
                {

                }
  
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            ////////////////////////////////////// Inserting New Items in database.tableB from database.tableA.
            string regString = "datasource=localhost;port=3306;username=root;password=1234";
            string regQuery = "INSERT INTO databse.tableB(Id,Items,Stock) VALUES ('" + this.idTxt.Text + "','" + this.itemsTxt.Text + "','" + this.brStockTxt.Text + "');";
            MySqlConnection regData = new MySqlConnection(regString);
            MySqlCommand regCommandData = new MySqlCommand(regQuery, regData);
            MySqlDataReader regReader;

            try
            {
                regData.Open();
                regReader = regCommandData.ExecuteReader();
                MessageBox.Show("Registered Data..");
                this.Hide();
                while (regReader.Read())
                {
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

//here's the another form called return form
 public Form3()
        {
            InitializeComponent();
            load_table();
        }
        //load_table to automatically loaded a data in datagridview from database.TableB 
         void load_table()
        {
            string regString = "datasource=localhost;port=3306;username=root;password=1234";
            MySqlConnection regData = new MySqlConnection(regString);
            MySqlCommand regCommandData = new MySqlCommand("Select * From database.tableB;", regData);

            try
            {
                MySqlDataAdapter sda = new MySqlDataAdapter();
                sda.SelectCommand = regCommandData;
                DataTable dbDatabase = new DataTable();
                sda.Fill(dbDatabase);
                BindingSource bSource = new BindingSource();

                bSource.DataSource = dbDatabase;
                dataGridView1.DataSource = bSource;
                sda.Update(dbDatabase);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

        }

    //If you choose or selecting an items from datagridview it will show the data directly in a given textbox

        private void dataGridView1_CellClick(object sender, DataGridViewCellEventArgs e)
        {
            this.rIdTxt.Text = dataGridView1.SelectedRows[0].Cells[1].Value.ToString();
            this.rItemsTxt.Text = dataGridView1.SelectedRows[0].Cells[2].Value.ToString();
            this.rStockTxt.Text = dataGridView1.SelectedRows[0].Cells[3].Value.ToString();
        }

//this is the return items button
//i added a delete button so if you click the return button the items you chosen from the datagridview that came from database.TableB will be deleted

        private void rBtn_Click(object sender, EventArgs e)
        {
            try
            {
                string modConnection = "datasource=localhost;port=3306;username=root;password=1234";
                string modQuery = "Delete from database.tableB Where Id = '" + this.rIdTxt.Text + "';";

                MySqlConnection modData = new MySqlConnection(modConnection);
                MySqlCommand modCommandData = new MySqlCommand(modQuery, modData);
                MySqlDataReader MyReader;

                modData.Open();
                MyReader = modCommandData.ExecuteReader();
                MessageBox.Show("Registered Data..");

                this.Hide();
                while (MyReader.Read())
                {

                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

            //Here's the problem what code should i use to perform a addition
the value i return from database.tableB to add it to database.TableA??

//lets say there's 3 available stock for BookA borrow(database.TableA) and i borrow 2 stocks from it, so the borrow(database.tableA) stock remaining will be 1, and the 2 stocks i borrowed will be added in return(database.tableB) with name BookA and the 2 stocks value i borrowed, and from return(database.tableB) i return the BookA with 2stocks that i borrowed, and added it again database.tableA

//can anyone help me please???

            try
            {
                string modConnection = "datasource=localhost;port=3306;username=root;password=1234";
                string modQuery = "UPDATE database.tableA SET Id = '" + this.rIdTxt.Text + "',Items = '" + this.rItemsTxt.Text + "',Stock = '" + (rStockTxt.Text+??????) + "' Where Id = '" + this.rIdTxt.Text + "';";

                MySqlConnection modData = new MySqlConnection(modConnection);
                MySqlCommand modCommandData = new MySqlCommand(modQuery, modData);
                MySqlDataReader MyReader;

                modData.Open();
                MyReader = modCommandData.ExecuteReader();
                MessageBox.Show("Registered Data..");

                this.Hide();
                while (MyReader.Read())
                {

                }

            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }



        }