Only change required it to modify your config.php
Change your WEB_PATH to reflect the https URL
From
define('WEB_PATH', 'http://yourdomain.com/');
To
define('WEB_PATH', 'https://yourdomain.com/');
With this change, all the internal links to your website will have the HTTPS proto.
SSL has nothing to do with Osclass or any other script you could run on your webserver, SSL is handle by the server (apache, nginx,...). I don't use CPanel, nor know about that plugin. Let's encrypt gives you a free SSL certificate that last 4 months (IIRC), the plugin may renew the certificate automatically (I imagine). Now, it depends on how you have configured your server and what you want to achieve, if you want to allow both HTTP and HTTPS connections, you need to modify a little more your config.php file.
Something like that will allow both connection
// this is in case you have a proxy and the connection proxy<-->server is not secure
if(isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && strtolower($_SERVER['HTTP_X_FORWARDED_PROTO'])=='https') {
$_SERVER['HTTPS'] = "on";
}
// allow both connection
if(isset($_SERVER['HTTPS']) && ($_SERVER['HTTPS']=='on' || $_SERVER['HTTPS']==1)) {
define('WEB_PATH', 'https://yourdomain.com/');
} else {
define('WEB_PATH', 'http://yourdomain.com/');
}
Warning: It depends on which server are you running, they may or may not send you the $_SERVER['HTTPS'] (some send a "1" value, others send a "on" value)
If you for example use nginx as a proxy, and the connection nginx-apache is not secure, $_SERVER['HTTPS'] will be empty/null but $_SERVER['HTTP_X_FORWARDED_PROTO'] will have a http/https value depending on which connection reach the proxy.
