There are literally dozens of ways to do this kind of thing. You could rewrite your website in any of the wide variety of server side languages, whether it's a scripting language like PHP or a compiled language like C# or Java.
You don't have to go that drastic, though. You can simplify it by including references in your pages. This can be a direct usage of another HTML file, or it can be a JavaScript file. The link below shows how to do the HTML file linkage using JQuery, as well as JavaScript. JavaScript doesn't have to be difficult and it can be referenced from all your pages, so it can have all the things in one file that you want to change, allowing you to change one file and it reflect on all your other pages.
HTML include with JQuery: (if you aren't already using JQuery, I'd hesitate to use a whole library to do one function that can be done in vanilla JS just as easily)
a.html:
<html>
<head>
<script src="jquery.js"></script>
<script>
$(function(){
$("#includedContent").load("b.html");
});
</script>
</head>
<body>
<div id="includedContent"></div>
</body>
</html>
b.html:
<p>This is my include file</p>
Or JavaScript:
a.html:
<html>
<body>
<h1>Put your HTML content before insertion of b.js.</h1>
...
<script src="b.js"></script>
...
<p>And whatever content you want afterwards.</p>
</body>
</html>
b.js:
document.write('\
\
<h1>Add your HTML code here</h1>\
\
<p>Notice however, that you have to escape LF's with a '\', just like\
demonstrated in this code listing.\
</p>\
\
');
Include another HTML file in a HTML file
If you are using PHP already, there's a good chance you can do it with the PHP you're already using by reading from a text file and inserting the HTML text into the page you are generating.
<?php
// do php stuff
include('fileOne.html');
// or //
readfile('fileTwo.html');
?>
how to embed html files in php code?