To use an 'outside' menu file, create a .txt file and put all your menu contents in it. Then, on each page where you want to include the menu, use
Code:
<!--#include file="menu.txt" -->
Optionaly, if you use PHP,
Code:
<? include("menu.txt"); ?>
Learn more here
This way, when you want to change your menu, all you have to do is edit one page and it will display on all the pages where you run that include statement.