I can't Update my records.

Hi Chinnasamy

I figured out the issue.

Make the below changes in your Default4.aspx page's Page_Load event.

Default4.aspx

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
       {
//Here I retrieve the session variable from previous page

        lblusername.Text = Session["username"].ToString();
       SqlConnection con = new SqlConnection("Data Source=KISHORE-PC;Initial Catalog=kavi;User ID=sa;Password=kishore");
        con.Open();
        SqlCommand cmd = new SqlCommand("select * from userreg where username='"+lblusername.Text+"'", con);
        SqlDataReader dr = cmd.ExecuteReader();
        if (dr.Read())
        {
            txtusername.Text = dr.GetString(0).ToString();
            txtpwd.Text = dr.GetString(1).ToString();
            txtage.Text = dr.GetString(2).ToString();
            txtmobile.Text = dr.GetString(3).ToString();
        }
      }
    }