Model有一个字段TotalCount。单击按钮时，如何更改它的值？

<input id="Qty" type="text" style="width:25px;" 
     value="@Model.TotalCount" />

    var pageKoModel;
    var pageKoModelData = @Html.Raw(Json.Encode(Model));
    $(document).ready(function () {
        pageKoModel = new Object();
        ko.mapping.fromJS(pageKoModelData, {}, pageKoModel);
        ko.applyBindings(pageKoModel);
    });