Is there a way to replace inside the DOM of a page using the replace() in javascript

In the source code I want to replace:

<div class="topbar">Bookmark Us</div>

to

<div class="topbar"><span class="larger-font">Bookmark Us</span></div>

When a Google Chrome extenstion is on the matched website of a URL and it will do the above.

Any page that matches:

http://www.domain.com/support.php

Thanks.