update records using textbox value

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C#

I have retrieve data from datatable to textbox and i have created a system navigation with a next button and its ok until here.
now i want to update data in this textbox and save them to the database.
i have code this but results me with an error.. everytime a change a value in a textbox.. all the records of the datatable store the same values
what have i done wrong???
here is my code:

namespace Csharp
{
    public partial class prova1 : System.Web.UI.Page
    {


        private DataTable GetData()
        {

            DataTable dt = new DataTable();


            SqlConnection connection = new SqlConnection(ConfigurationManager.ConnectionStrings["ProvaConnectionString"].ConnectionString);

            try
            {

                connection.Open();

                SqlCommand sqlCmd = new SqlCommand("Select * From prova", connection);
                SqlCommand sqlCmd2 = new SqlCommand("UPDATE Customer SET Code='" + txt_kodi.Value + "', Tel='" + txt_tel.Value + "', Mobile = '" + txt_mobile.Value + "'");

                SqlDataAdapter sqlDa = new SqlDataAdapter(sqlCmd);



                sqlDa.Fill(dt);

            }

            catch (System.Data.SqlClient.SqlException ex)
            {

                string msg = "Fetch Error:";

                msg += ex.Message;

                throw new Exception(msg);



            }

            finally
            {

                connection.Close();

            }

            return dt;

        }

        protected void Page_Load(object sender, EventArgs e)
        {

            if (!Page.IsPostBack)
            {
                txt_kodi.Disabled = true;
                txt_persh.Disabled = true;
                txt_pershholl.Disabled = true;
                txt_tel.Disabled = true;
                txt_mobile.Disabled = true;

                DataTable dt = GetData();
                Hidden_id.Value = "0";
                if (dt.Rows.Count > 0)
                {

                    // Populate the TextBox with the first entry on page load

                    txt_kodi.Value = dt.Rows[0]["Code"].ToString();

                   

                    txt_tel.Value = dt.Rows[0]["Tel"].ToString();

                    txt_mobile.Value = dt.Rows[0]["Mobile"].ToString();





                    //Then we store the DataTable in Session so that we will NOT

                    //query the DB on every postbacks

                    Session["dt"] = dt;

                }



            }

        }



        protected void btn_next_Click(object sender, EventArgs e)
        {

            int rowIndex = 0;

            if (Hidden_id.Value.Trim() != null)
            {
                rowIndex = Convert.ToInt32(Hidden_id.Value) + 1;



                if (Session["dt"] != null)
                {

                    DataTable dt = (DataTable)Session["dt"];

                    if (rowIndex < dt.Rows.Count)
                    {

                        //get the next row entry on Button Click by setting the Row Index
                        txt_kodi.Value = dt.Rows[rowIndex]["Code"].ToString();

                     

                        txt_tel.Value = dt.Rows[rowIndex]["Tel"].ToString();

                        txt_mobile.Value = dt.Rows[rowIndex]["Mobile"].ToString();

                        Session["dt"] = dt;
                        Hidden_id.Value = rowIndex.ToString();



                    }
                    else

                    { Response.Write("Last record!"); }

                }

            }
        }
 protected void btn_modify_Click(object sender, EventArgs e)
        {
            SqlConnection connection = new SqlConnection(ConfigurationManager.ConnectionStrings["ProvaConnectionString"].ConnectionString);

            SqlCommand sqlCmd2 = new SqlCommand("UPDATE prova SET Code='" + txt_kodi.Value + "', Tel='" + txt_tel.Value + "', Mobile = '" + txt_mobile.Value + "'");

            SqlDataAdapter sqlDa = new SqlDataAdapter(sqlCmd2);

            SqlCommandBuilder cmdBuilder = new SqlCommandBuilder();

            cmdBuilder = new SqlCommandBuilder(sqlDa);


            int rowIndex = 0;

            if (Hidden_id.Value.Trim() != null)
            {
                rowIndex = Convert.ToInt32(Hidden_id.Value);



                if (Session["dt"] != null)
                {

                    DataTable dt = (DataTable)Session["dt"];
                    rowIndex = 0;

                    System.Data.DataRow dRow = dt.Rows[rowIndex];

                    dRow["Code"] = txt_kodi.Value;
                   
                    dRow["Tel"] = txt_tel.Value;
                    dRow["Mobile"] = txt_mobile.Value;
                    connection.Open();
                    sqlCmd2.Connection = connection;
                    sqlCmd2.ExecuteNonQuery();
                    connection.Close();
                
                    Session["dt"] = dt;
                    Hidden_id.Value = rowIndex.ToString();
                    Response.Write("data save!");
                }
            }
        }
    }