Update author field

 var ctx = new SP.ClientContext.get_current();
      var list = ctx.get_web().get_lists().getByTitle("Test");
      var item = list.getItemById(26);
      item.set_item("Author", 764);
      item.update();
      ctx.executeQueryAsync(
         function () {console.log("Done") },
      function () {console.log("Error")}
);