If you only have HTML files with no preprocessor, CMS (Content Management System) or the like, you are most likely out of luck and you will have to edit all files individually. If I had to do so, I would most likely write a quick and dirty script (NodeJS in my case) which searches all files in a directory for a given string and replaces that string with another given string (alternatively use DOM tools for a better script and better results).
Big companies do not just write HTML files, they usually use a CMS or HTML generator. For small sites, PHP (PHP Hypertext Preprocessor) is a nice tool/language to write separate partials and include them into your content sites. If you do not have the server-capabilities to even run PHP, WebComponents might be the solution for you (at the moment, they still need polyfills!).
PHP Example
<!-- footer.html -->
<div class=footer>
(c) Rajat
</div>
<!DOCTYPE HTML>
<!-- Some other file -->
<title>Rajat's Site</title>
<div class=content>Welcome to my website! Some text here<div>
<?php include('footer.html'); ?>
Polymer (WebComponents) Example
<!-- my-footer.html -->
<link rel="import" href="bower_components/polymer/polymer.html">
<dom-module id="my-footer">
<!-- Defines the element's style and local DOM -->
<template>
<style>
:host {
display: block;
}
</style>
<div class=footer>(c) Rajat</div>
</template>
<!-- Creates the element's prototype and registers it -->
<script>
Polymer({
is: 'my-footer'
});
</script>
</dom-module>
<!DOCTYPE HTML>
<!-- Main file -->
<title>Rajat's Site</title>
<script src="polygit.org/components/webcomponentsjs/webcompone…"></script>
<link rel="import" href="webcomponents/my-footer.html">
<div class=content>Welcome to my website! Some text here<div>
<my-footer></my-footer>