You will need to edit pages (that probably have a
.aspx extension) that have both html and some server side code mixed together but you probably won't need to change the server side piece much, maybe just where it appears. These are plain text files that you can edit with any editor, just like .html files. You may want to use an IDE or you may find it easier to just edit the pages with a basic editor for your OS platform (Notepad, emacs, textmate, vi, etc).
<title>The current time</title>
The server's current time:<br />
The HTML you should be comfortable with. The stuff within the
<% %>'s is the server side code. In this case you can guess from the surrounding html code that
Response.Write Now() puts out the current time.
You will want to 'see' what the pages look like you will need to either run a local server or have a test server that you can push changes to to see their effect.
Much much more at: http://en.wikipedia.org/wiki/Active_Server_Pages