Pass values from table to update

I have table , which is filled by the bank and can be changed via ajax , changed only in table and unchanged in the bank. What happens, is that as the data can be changed, I need to send the changed new data to save, I'm passing it this way:

$('#Editar').submit(function (e) {
e.preventDefault();
var id = document.getElementById("nfseid").value;
var valores = [];
$('.item').each(function () {
    var entidade = {
        Id: parseInt($(this).children()[10].innerText),
        Codigo: ($(this).children()[1].innerText),
        Descricao: ($(this).children()[2].innerText),
        UnMedida: ($(this).children()[3].innerText),
        Qtd: ($(this).children()[4].innerText),
        VlrUnitario: ($(this).children()[5].innerText),
        Deducao: ($(this).children()[6].innerText),
        DescCondicionado: ($(this).children()[7].innerText),
        DescIncondicionado: ($(this).children()[8].innerText),
        Total: ($(this).children()[9].innerText),
    };
    valores.push(entidade);
});
var obj = {};
obj.valores = valores;
var form = this,
    $form = $(form);

$.ajax({
    url: "/NFSe/SalvaNFSItens",
    type: 'POST',
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    data: JSON.stringify(obj),
    success: function (data) {
        if (data.resultado == true) {
            if ($('#Editar').valid()) {
                $form.off('submit').submit();
            }
        }
    }
})
})

However, how can I compare what has been changed to make update , insert , or delete ?

 [HttpPost]
    public async Task<IActionResult> SalvaNFSItens([FromBody] ObjetoNFSeItens obj)
    {
        ApplicationUser user = await _userManager.GetUserAsync(User);

        var result = false;
        foreach (var item in obj.valores)
        {
            var valor = db.NFSeItens.Where(a => a.Id == item.Id).Single();
            valor.ProdutoId = item.ProdutoId;
            valor.qtd = item.qtd;
            valor.UnMedida = item.UnMedida;
            valor.ValorUnitário = item.ValorUnitário;
            valor.ValorTotal = item.ValorTotal;
            valor.Descricao = item.Descricao;
            valor.DescontoIncondicionado = item.DescontoIncondicionado;
            valor.DescontoCondicionado = item.DescontoCondicionado;
            valor.Deducao = item.Deducao;

            db.Update(valor);
            db.SaveChanges();
            result = true;
        }
        return Json(new { resultado = result });

    }

How to do this in foreach ?? I am trying to do this, however it is returning error in this line: var valor = db.NFSeItens.Where(a => a.Id == item.Id).Single();