DOM

Document Object Model

all elements in a html page can have an id and be altered .

Here's how to grab the element by the id and change it using javascript:

<a id="link" href="http://www.yahoo.com">testing</a>

<script type="text/javascript">
 link=document.getElementById('link');
  link.href="http://msdn.com";
</script>

Now when you see the link it should link to http://msdn.com even though the original href points to www.yahoo.com