Where is the mistake in my source?

 using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
using System.Data.Common;
using System.Data.OleDb;


namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        SqlConnection conn;
        SqlDataAdapter da;
        DataSet ds;
        SqlCommandBuilder cmb;

        public Form1()
        {
            InitializeComponent();

            SqlConnection conn = new SqlConnection("Data Source=MACHINE;Initial Catalog=JobDetailsDB;Integrated Security=True");
            conn.Open();
            SqlDataAdapter da = new SqlDataAdapter("Select * FROM JobDetailsDB", conn);
            DataSet ds = new DataSet();
            SqlCommandBuilder cmb = new SqlCommandBuilder(da);
            
            dg.DataSource = ds.Tables["JobDetailsTbl"];

           
        }

        private void Form1_Load(object sender, EventArgs e)
        {

            

        }

        private void btnUpdate_Click(object sender, EventArgs e)
        {
            da.UpdateCommand = cmb.GetUpdateCommand();

            da.Update(ds);
        }
      
    }

}

 da.UpdateCommand = cmb.GetUpdateCommand();