In the body, lines like:

<iframe id="some_iframe" name="some_iframe" style="vertical-align: top" frameborder="0" scrolling="no" src="some_iframed_page.html" onload="ifr_resize(this.id)" ></iframe>


<div style="width: 100%">
<iframe id="some_iframe2" name="some_iframe2" style="vertical-align: top" frameborder="0" scrolling="no" src="some_iframed_page2.html" onload="ifr_resize(this.id)" ></iframe> 
</div>

<div style="width: 60%; border: 1px solid red">
<iframe id="some_iframe3" name="some_iframe3" style="vertical-align: top" frameborder="0" scrolling="no" src="some_iframed_page3.html" onload="ifr_resize(this.id)" ></iframe> 
</div>

etc.

You can change the iframe's URL from inside the parent page by doing:
<a href="some_other_iframed_page.html" target="some_iframe">Change URL of first iframe</a>
<a href="some_other_iframed_page2.html" target="some_iframe2">Change URL of second iframe</a>
etc.
Changing the URL from inside an iframed page can de done without target.