Picking out Just JSON Data Returned from ASP.NET MVC3 controller Update
[HttpPost]
    public JsonResult Update(President data)
    {
        bool success = false;
        string message = "no record found";
        if (data != null && data.Id > 0)
        {
            using (var db = new USPresidentsDb())
            {
                var rec = db.Presidents.FirstOrDefault(a => a.Id == data.Id);
                rec.FirstName = data.FirstName;
                db.SaveChanges();
                success = true;
                message = "Update method called successfully";
            }
        }

        return Json(new
        {
            data,
            success,
            message
        });
    }

rec.FirstName = data.FirstName ?? rec.FirstName;

if (data.FirstName != null)
   rec.FirstName = data.FirstName
.
.
.

public static void CopyOnlyModifiedData<T>(T source, ref T destination)
{
    foreach (var propertyInfo in source.GetType().GetProperties())
    {
        object value = propertyInfo.GetValue(source, null);
        if (value!= null && !value.GetType().IsValueType)
        {
            destination.GetType().GetProperty(propertyInfo.Name, value.GetType()).SetValue(destination, value, null);
        }
    }
}

CopyOnlyModifiedData<President>(data, ref rec);