Dear Jitendra,
Actually if you are using knockoutJS, this means that your design is centered around the model. Thus, if your autocomplete returns some value - most logically is to change value of the model, rather than it's presentation in input.
If you still insist on changing value of the input, just force triggering "change" event after the operation:
document.getElementById("changeit").addEventListener("click", function(){
document.getElementById("SymptomBox").value = "new value";
$("#SymptomBox").trigger("change");
});
Simple example:
http://jsfiddle.net/voronenko/QATTg/[
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