To set your .html and .htm files to be parsed as PHP, you can use a .htaccess file. If this file does not exist in the directory where you want to apply the change, you should create it first. You can easily create and edit the file via the File Manager section of the Control Panel.Add this line of code to the file:
AddHandler application/x-httpd-php .html .htm
If you want to use a different PHP version than the default one for the server, you should refer to the PHP Version article for more details.
Here is the format that you should use to utilize PHP 5.6 for example:
AddHandler application/x-httpd-php56 .php