Create a new blank page called jstextcontent.html.  Put this into the head element:

<script type="text/javascript">
    function changeContent(){
        document.getElementById('something').textContent='Now it is not.';
    }
</script>

Now put this inside the body:

<p id="something" onmouseup="changeContent();">This is a paragraph created by HTML.</p>

Save and view.  Click on the paragraph.

This is one area where there has been some disagreement amongst browsers so on old browsers if this doesn't work try innerHTML instead of textContent.