Here's the plan:

After button is clicked, the new site is opened in new window. But, the url of
the site has to be read from external file (for instance text file) placed in
the same directory as flash files. My client needs to frequently change urls
assigned to the button by editing just text file containing url, not by editing
whole *.fla file. How can I do this? What script should I write ?
I use Flash 8 Pro.

Yes, you can certainly do this...in several different ways, as a matter of
fact. Searching the local help files in Flash is always the best place to
start. Here's the LiveDocs link on loading external files. That should get you
started:

http://livedocs.macromedia.com/flash/8/main/00000752.html

Good luck!

Try this:

loadVariablesNum("myURL.txt", 99);

myButton.onRelease = function() {
this.getURL(myURLvar, "_blank");
}

The myURL.txt file should look like this:

myURLvar="http://www.mywebsite.com"

You could also check for a successful load of the file. See LiveDocs for more
help there:
http://livedocs.macromedia.com/flash/8/main/00001742.html

Good luck!