How to update values in the database from the form

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ASP.NET

MySQL

Helo

I design the dhtml form and displaying values from database when updating that display value they are not affecting that particular record .They are saving in new record.

My table columns are Id,Name,Duration,Year,Comments
My view file is

myForm.attachEvent("onButtonClick", function (name, command) {

            if (name == "Update" && command == "Update") {
            
            
                $.ajax({

                    url: '/Home/Update',

                    data: JSON.stringify({
                        
                        Name: myForm.getItemValue("Name"),
                        Duration: myForm.getItemValue("Duration"),
                        Year: myForm.getItemValue("Year"),
                        Comments: myForm.getItemValue("Comments")

                    }),
                    
                    cache: false,
                    dataType: "json",
                    success: function (str) {
                        alert(str.st);
                       

                    },
                    type: 'POST',
                    contentType: 'application/json; charset=utf-8'
                });
               

            }

        });

In my controller

public ActionResult Update(int Id,string Name, int Duration, int Year, string Comments)
        {

            Training d = new Training();

            var st = (from i in dbContext.Trainings
                      where i.Id==Id
                      select i);

            if (d.Id==Id)
            {

                d.Name = Name;
                d.Duration = Duration;
                d.Year = Year;
                d.Comments = Comments;
            }
            try
            {
                this.dbContext.Add(d);
                this.dbContext.SaveChanges();
            }
            catch
            {
                return View();
            }

            var str = new { st = "Successfully updated" };
            return Json(str, JsonRequestBehavior.AllowGet);
        }

If i was updating the operation is not successful if any one knows please help me