Finding Specific HTML And Replacing It

I'm trying to create a piece of code that will check a webpage and replace any instances of a specific line of HTML with another line of HTML. The code I have so far doesn't work, but I have a feeling that's down to my awful Javascript skills! For example, in the following code I'd want to replace all instances of
<li><a href="index.pdf">Index</a></li> with <li><a href="contents.pdf">Contents</a></li>

<div id="downloads">
<p>Please click on your downloads below:</p>
<ul>
<li><a href="index.pdf">Index</a></li>
<li><a href="chapter1.pdf">Chapter 1</a></li>
<li><a href="chapter2.pdf">Chapter 2</a></li>
<li><a href="chapter3.pdf">Chapter 3</a></li>
<li><a href="credits.pdf">Credits</a></li>
</ul>
</div>

The Javascript I have so far is:
<script type="text/javascript">
var aEls = document.getElementsByTagName('a');
for (var i = 0, aEl; aEl = aEls[i]; i++) {
aEl.href = aEl.href.replace('index.pdf','contents.pdf');
}
Obviously this is meant to just replaces the a href elements, but I got a bit stuck after this!