<select id="mydropdown">
<option value="item 1">item 1</option>
<option value="item 2">item 2</option>
<option value="item 3">item 3</option>
</select>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>
// Id of Select Tag, Value of Option, Use as Selected
$("#mydropdown").val("item 2").attr('selected',true);
</script>