How to update selected row in gridview?

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I have a TextBox with id=txtPlace and a gridview. If a user selects a row an put some Information in the TextBox and click on save. The Information should replace older records in the database(SQL)

But It does not work. When I click on save, nothing happens.

protected void btnSave_Click(object sender, EventArgs e)
   {
       var id = GridView2.SelectedDataKey.Values[0].ToString();
       string constr = ConfigurationManager.ConnectionStrings["con"].ConnectionString;
       using (SqlConnection conn = new SqlConnection(constr))
       {

           string query = "UPDATE user SET place= @place where ID = @D ";
           SqlCommand cmd = new SqlCommand(query, conn);
           cmd.Parameters.AddWithValue("@place", txtPlace.Text);
           cmd.Parameters.AddWithValue("@ID", id);

       }
   }