<select id="mySelect">
<option value="1">Yes</option>
<option value="2">No</option>
</select>
<input type="button" id="demo" value = "Demo" />
<script type="text/javascript">
$("#demo").live("click", function () {
//Clear already selected option
$("#mySelect option").removeAttr("selected");
//Set option with value 2 as selected
$("#mySelect option[value=2]").attr("selected", "selected");
});
</script>