change user password

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C#

ASP.NET

C#4.0

i am developing a web app in visual studio using C# and sql server.
I want to create a page that the user should change his password.
i have tried this but no successs
pls help me find the error

namespace Csharp
{
    public partial class change : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (Session["new"] != null)
            {
                txt_username.Disabled = true;
                txt_username.Value = Session["username"].ToString();
            }
        }

       
        protected void btn_submit_Click(object sender, EventArgs e)
        {

            if (txt_newpass.Value.Length < 6)
            {
                Response.Write("<script language=Javascript>.....!</script>");
            }

            if (txt_newpass.Value != txt_newpass2.Value)
            {
                Response.Write("<script language=Javascript>......!</script>");
            }

           
            
            
            
            string pass;

            DataTable dt = new DataTable();
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionStringusersregj"].ConnectionString);
            conn.Open();

           
            string querysel = "Select * from Users where UserName='" + Session["username"].ToString() + "' ";
            SqlCommand kom = new SqlCommand(querysel, conn); 

            SqlDataAdapter sqlDa = new SqlDataAdapter(kom);

            sqlDa.Fill(dt);

            if(dt.Rows.Count>0)
            {
                pass = dt.Rows[0]["Password"].ToString();
                if (pass == txt_password.Value)
                {

                    string cod = "UPDATE Users set Password='"+ txt_newpass.Value +"' where UserName='" + Session["new"].ToString() + "'";
                    SqlCommand cmd = new SqlCommand(cod, conn);
                    cmd.ExecuteNonQuery();
                }
                else
                {
                    Response.Write("<script language=Javascript>Old pass is incorredt</script>");
                }
            }
            conn.Close();
        }
    }
}