<select id="target">
  <option value="1">...</option>
  <option value="2">...</option>
</select>

$("#target").val($("#target option:first").val());

$("#target").prop("selectedIndex", 0);

// remove "selected" from any options that might already be selected
$('#target option[selected="selected"]').each(
    function() {
        $(this).removeAttr('selected');
    }
);


// mark the first option as selected
$("#target option:first").attr('selected','selected');