Stack Overflow answer Jquery
by lopisan
HTML
<select id="a">
<option value="1">one</option>
<option value="2">two</option>
<option value="3">three</option>
</select>
<select id="b">
<option value="1">one</option>
<option value="2">two</option>
<option value="3">three</option>
</select>
<button id="switch">Switch</button>
JavaScript
$("#switch").click(function(){
var valA = $("#a").val();
var valB = $("#b").val();
$("#a").val(valB);
$("#b").val(valA);
});