Как вариант...
<!DOCTYPE html>
<html>
<head>
<script src='http://code.jquery.com/jquery-latest.js'></script>
<!--
<link rel='stylesheet type=text/css href=tmp.css' />
-->
<style type='text/css'>
</style>
<script type='text/javascript'>
$(function (){
$('.reduce').click(function (){
var o=$(this).next();
var val=parseInt(o.val());
if (val>0) {
o.val(val-1);
};
});
$('.add').click(function (){
var o=$(this).prev();
var val=parseInt(o.val());
o.val(val+1);
});
});
</script>
</head>
<body>
<div id="qty-spinner" class="qty">
<i data-helper="reduce" class="qty__helper reduce">-</i>
<input type="text" id="product_qty" class="item-qty" value="4">
<i data-helper="add" class="qty__helper add">+</i>
</div>
</body>
</html>
</html>