try this :
@Html.DropDownList("Sortby", new SelectListItem[] { new SelectListItem()
{ Text = "Newest to Oldest", Value = "0" }, new SelectListItem()
{ Text = "Oldest to Newest", Value = "1" }},
new { onchange = "document.location.href = '/ControllerName/ActionName?id=' + this.options[this.selectedIndex].value;" })