Update specific field using Entity Framework in ASP.Net Core

How to update specific field not all using Core

I am calling getting data using below code.

public IActionResult Edit(int? Id)
{
    try
    {
        var edititem = _context.ItemMasterFiles.Find(Id);

        if (edititem != null)
        {
            var item = new ItemMasterFile();
            {
                item.CodeItem = edititem.CodeItem;
                item.Descriptionitem = edititem.Descriptionitem;
                item.SecId = edititem.SecId;
                item.Cid = edititem.Cid;
                item.Weight = edititem.Weight;
                item.BaleSize = edititem.BaleSize;
            };

            var sectionList = (from s in _context.Sections
                                select new SelectListItem()
                                {
                                    Text = s.Secnam,
                                    Value = s.SecId.ToString()
                                }).ToList();

            sectionList.Insert(0, new SelectListItem()
            {
                Text = "----Select----",
                Value = string.Empty
            });

            item.Listofsections = sectionList;
            var categoryList = (from s in _context.Catagories
                                select new SelectListItem()
                                {
                                    Text = s.Cname,
                                    Value = s.Cid.ToString()
                                }).ToList();

            categoryList.Insert(0, new SelectListItem()
            {
                Text = "----Select----",
                Value = string.Empty
            });

            item.Listofcategory = categoryList;
            return View(item);
        }
        else
        {
            TempData["errorMessage"] = "Item Detail is not available..";
            return RedirectToAction("ItemList");
        }
    }

    catch (Exception ex)
    {
        TempData["errorMessage"] = ex.Message;
        return RedirectToAction("ItemList");
    }
}

And updating specific field as below code

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Edit(ItemMasterFile item)
{
    if (ModelState.IsValid)
    {
        var CheckitemName = _context.ItemMasterFiles.Where(u => u.Descriptionitem == u.Descriptionitem && u.CodeItem != u.CodeItem).FirstOrDefault();

        if (CheckitemName == null)
        {
            var edititem = new ItemMasterFile()
            {
                CodeItem = item.CodeItem,
                Descriptionitem = item.Descriptionitem,
                Weight = item.Weight,
                BaleSize = item.BaleSize
            };
            _context.Entry(edititem).State = EntityState.Modified;
            _context.SaveChanges();
            return RedirectToAction("ItemList");                    
        }
    }
    else
    {
        ModelState.AddModelError("Descriptionitem", "Already Exist");
    }
    return View();
}

 But it is update all fields, which should not be.

CodeItem = item.CodeItem,
Descriptionitem = item.Descriptionitem,
Weight = item.Weight,
BaleSize = item.BaleSize