<script type="text/javascript">

    var clicks = 0;

    function onClicka(){
        clicks += 1;
        document.getElementById("clicks").innerHTML = clicks;
    };

    function onClickb(){
        clicks -= 1;
        document.getElementById("clicks").innerHTML = clicks;
    };

    </script>

<img src="images\pfeilrunter.png" onClick="onClickb()" width="16px" height="16px">
<img src="images\pfeilhoch.png" onClick="onClicka()" width="16px" height="16px">

<form  action="hn_l2.php" method="post"  >
<input type="text" name="daten_aus_l1" id="clicks"><br>
<input type="submit" value="send mich ab">
</form>