Edit in JSFiddle

$("document").ready(function() {
    $("#b1").click(function() {
        $(".selDiv select.opts option").each(function() {
            $(this).removeAttr("selected");
        });
        var optSelValue = $("#txt").val();
        $(".selDiv select.opts option[value=\"" + optSelValue + "\"]").attr("selected", "selected");
    });
});

<div class="selDiv">
        <select class="opts" multiple='multiple'>
                <option selected value="DEFAULT">Default</option>
                <option value="SEL1">Selection 1</option>
                <option value="SEL2">Selection 2</option>
        </select>
</div>
<input type='text' id='txt' />

<input type='button' id='b1' value='change option selected' />