How to save(update) data back to database from datagrid C# WPF

See more:

C#

SQL

WPF

DataGrid

Hi everyone. I am working with a small application(inventory), where a have some buttons a datagrid that retrieves data from SQL Database. When i retrieve data from SQL in datagrid i have four columns like this for example
Code Product Qty Price Total
123456789 Coke 1 0.8 0.8

Total i a expression column(Qty * Price)
What i want to do that when the user change the qty from 1 to 2 and he hits button save the data from datagrid should be saved to another table in database, also the qty of selected product should deduct for example from 40 to 38.
Could anyone help me, a am stucked here

What I have tried:

if (e.Key == Key.Enter)
{
    SqlConnection con = new SqlConnection("Server = localhost;Database = Bilanc; Integrated Security = true");
    SqlCommand cmd = new SqlCommand("Product", con); // Using a Store Procedure.
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("Barcode", txtcode.Text);




    dtg.ItemsSource = dataTable.DefaultView;//Set the DataGrid ItemSource to this new generated DataTable

    con.Open();//Open the SQL connection

    SqlDataReader reader = cmd.ExecuteReader();//Create a SqlDataReader

    while (reader.Read())//For each row that the SQL query returns do
    {
        DataRow dr = dataTable.NewRow();//Create new DataRow to populate the DataTable (which is currently binded to the DataGrid)
        dr[0] = reader[0];//Fill DataTable column 0 current row (Product) with reader[0] (Product from sql)
        dr[1] = reader[1];
        dr[2] = reader[2];
        dr[3] = reader[3];



        dataTable.Rows.Add(dr);//Add the new created DataRow to the DataTable
        txtkodi.Text = "";

        object sumObject;
        sumObject = dataTable.Compute("Sum(Total)", "");
        txttot.Text = sumObject.ToString();

    }