partial refresh of listbox

[Jemuel]

I am having trouble on how will I solve this problem. I want only to refresh the value of listbox after adding a value from textbox using onclick() on an image.

<form id="UpdateDetails">
<input type="hidden" name="id">
<input id="adnew" type="text" autocomplete="off" name="addnew" class="xx">
<img id="img" onclick="addnew()" src="images/icons/add-icon.png" style='padding:0 0 0 10px;height:15px;width:15px;cursor:pointer;' title='Add New' />
<span id="checkStatus"></span><br/>
<select multiple>
            <?php
                $id=$_POST['id'];
                $sq = mysql_query("SELECT testVal FROM x_file WHERE id='$id'");
                while($q = mysql_fetch_array($sq)){
                    echo "<option value=".$q['testVal'].">".$q['testVal']."</option>";                }
            ?>
        </select>
</form>
<script>
function addnew(){
$.ajax({
            type: 'post',
            url: 'addnew.php',
            data: $('#UpdateDetails').serialize(),
            success: function (info) {
            $('#checkStatus').fadeIn(250).css('color', '#017c04').html(info).delay(1000).fadeOut(500);
            clearInput();
            }
          });
 }
</script>