Entity Data Framework - Update a record through the primary key. EntityKey (namespace System.Data)

  ContraEntities contraEntityCtx = new ContraEntities(ConfigurationManager.ConnectionStrings["ContraCSEntities"].ToString());

  int ID_SpidBustaPagaDettaglio = Convert.ToInt32(lbl_ID_SpidBustaPagaDettaglio_Save.Text);

  bpd = contraEntityCtx.tbl_SpidBustaPagaDettaglio.Where(a => a.ID_SpidBustaPagaDettaglio == ID_SpidBustaPagaDettaglio).FirstOrDefault();

  bpd.Descrizione = txtDescrizione.Text; int IDTemplateItem = Convert.ToInt32(ddlItemTemplate.SelectedValue);           

  bpd.tbl_TemplateItemsReference.EntityKey = new EntityKey("ContraEntities.tbl_TemplateItems", "IDTemplateItem", IDTemplateItem); 

  contraEntityCtx.SaveChanges();