Try this:
<form id="Menu" method="post" action=""><input id="Height1" name="Height1" type="text" size="10" value="FUK!"></form>
<input type="button" onclick="Save()" style="color:ff0000" value="Plz work...">
<script type='text/javascript'>
function Save(){
document.getElementById("Menu").getElementById("Height1").value="OMFG WORKS!";
}
</script>
I had some problems, too, but the getElementById() function helped. Also you should not try to access Elements directly. User getElementByName(), getElementById() or getElementsByTag() functions.