<tbody>
                    @using (Html.BeginForm("Index2", "Cart", FormMethod.Post))
                        {
                            foreach (CartLine l in Model.Lines)
                            {
                                ...
                            }
                                 <tr>
                                     <td colspan="5" class="">
                                          <button id="btnUpdCart" type="submit" class="btn btn-base-4 pull-right">Обновить</button>
                                     </td>
                                 </tr>
                         }
</tbody>

      public ViewResult Index2()
        {
            return View(GetCart());
        }