I have answered my own question as nobody else wants to

To change the "search" button to a Fontawesome Magnifying glass

In folder: Includes/Languages/English

The File: "header.php"

Change the following:

define('HEADER_SEARCH_BUTTON','Search');

To the following:

define('HEADER_SEARCH_BUTTON','<i class="fa fa-search"></i>');

This will remove the word "search" and replace it with a Magnifying Glass only
You can still include the "search" word by using the following:

define('HEADER_SEARCH_BUTTON','<i class="fa fa-search"></i> Search');