When I make an IFRAME, how do I make a link above the IFRAME that changes the page displaying in the IFRAME?

<a href="abc.html" target="your_frame_name">Link</a>

make sure your iframe has the same name:

<iframe src="xyz.html" name="your_frame_name">You cannot view iframes</iframe>

target your links...add target="nameofyouriframe" to the <a> tag

this has been asked before, so you could have used the search to find the answer, and it is covered on the Frames page:
http://www.lissaexplains.com/frames.shtml
just so you know for future reference :)