I did a demo some time ago with this technic.
if you have main page
<frameset cols="100%,*
<frame src=src1 name=content
<frame src=myservlet
</frameset
and your content frame has a
<form name=form1
<input name=in1
to update it your servlet should generate the following javascript:
top.document.form1.in1.value="new value"
I use this example, since input text is a simpliest thing that you could have. If you need a static text to be changed,
you should wrap it into "span" with sertain id, so if
<span id=sp1
than to change the text:
top.document.getElementById("sp1").innerHTML="new value"
Note, that you will have a problem with doing span staff in NS4
it get more complicated, if need to change select's or add/remove some elements - I mean not just edit existing information.
