You have to use Kendo UI DropDownList select method (documentation in here).
Basically you should:
// get a reference to the dropdown list
var dropdownlist = $("#Instrument").data("kendoDropDownList");
If you know the index you can use:
// selects by index
dropdownlist.select(1);
If not, use:
// selects item if its text is equal to "test" using predicate function
dropdownlist.select(function(dataItem) {
return dataItem.symbol === "test";
});
JSFiddle example here