PashPP,
<!DOCTYPE html>
<html>
<head>
<title>test</title>
<script type="text/javascript">
function modify_qty(val) {
var qty = parseInt(document.getElementById("qty").value, 10);
var new_qty = qty + val;
document.getElementById("qty").value = new_qty;
return new_qty;
}
</script>
<style>
</style>
</head>
<body>
<form name="form">
<label>Quantity</label>
<input type="text" id="qty" value="0" />
<input type="button" value="-1" onClick="modify_qty(-1)">
<input type="button" value="+1" onClick="modify_qty(1)">
</form>
</body>
</html>