Hi,
Welcome to the forum.
You have two options:
1. Change your pages to PHP pages and use PHP includes. Check out my video on PHP includes. The problem with this approach, that you will need to change the pages from being .html pages to .php pages.
2. You can use perhaps something based on Ajax or even more simply, use an iframe to contain your menu.
Which solution to choose?
If I didn't have much direct traffic coming to the sub-pages of the site, I would just go ahead with PHP includes - they are overall the best option. Otherwise, I would look to using an iframe.
Stefan