Create an .htaccess file and upload it to the root of your public_html directory. Include the following lines...
RemoveHandler .html .htm
AddType application/x-httpd-php .php .htm .html
Because these extensions will be processed as PHP with the above command, changing the permission on those file types to 755 may be necessary.
If the above line does not work use the following:
AddType application/x-httpd-php5 .php5 .php4 .php .php3 .php2 .phtml .html .htm
or
AddType application/x-httpd-php .php5 .php4 .php .php3 .php2 .phtml .html .htm
If using CloudLinux lsapi /LiteSpeed API, use the following:
AddType application/x-httpd-lsphp .php5 .php4 .php .php3 .php2 .phtml .html .htm
AddHandler application/x-httpd-lsphp .php5 .php4 .php .php3 .php2 .phtml .html .htm