Update DB with updated Textbox Information in C#

Trying to update the first name of the student there is a textbox "FirstNameTextbox" information was loaded to it from the DB, when I change the information in the textbox and try to write the changes it read only the original data.So if it loaded "Craig" as the first name from the DB, i would edit and put "Chris" in the textbox, what happens is that Craig is written to the DB and not "Chris"

int stuID = getSqlStuID(IDNUMLabel.Text);
SqlConnection conn = new SqlConnection(GetConnectionString());
string sqlUpdateStudent = "Update tblStudent set fname = @fname where stuID = @stuID";
SqlCommand cmd = new SqlCommand(sqlUpdateStudent, conn);

conn.Open();

cmd.Parameters.AddWithValue("@stuID", stuID);
cmd.Parameters.AddWithValue("@fname", FirstNameTextbox.Text);

cmd.ExecuteNonQuery();

ErrorMessage.Text = "Success";


protected void Page_Load(object sender, EventArgs e)
{
    if (Session["User"] != null)
    {
        IDNUMLabel.Text = Session["User"].ToString();
        getStuData(Session["User"].ToString());
    }
    else
    {
        Response.Redirect("../Login/Login.aspx");
    }
}

private void getStuData(string id)
{
    SqlConnection conn = new SqlConnection(GetConnectionString());

    string sql = "Select fname, sname From tblStudent Where idnumber = '" + id + "' ";

    SqlCommand cmd = new SqlCommand(sql, conn);

    try
    {
        conn.Open();

        SqlDataReader selectedRecord = cmd.ExecuteReader();

        cmd.CommandType = CommandType.Text;

        while (selectedRecord.Read())
        {
            FirstNameTextbox.Text = selectedRecord["fname"].ToString();
            LastNameTextbox.Text = selectedRecord["sname"].ToString();
        }

        selectedRecord.Close();
    }
    catch (System.Data.SqlClient.SqlException ex)
    {

        //id = 0;
        //string msg = "Error reading Student ID";
        //msg += ex.Message;
        //throw new Exception(msg);
    }
    catch (Exception ex)
    {

    }
    finally
    {
        conn.Close();
    }
}