You can force an HTTPS connection on your website by adding these rules in your website's .htaccess file:
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
The .htaccess file needs to be located inside the site's document root folder. If your website is in a subfolder, then the .htaccess should be placed in the corresponding sub folder.
You can create or edit the .htaccess file either via FTP, or using it with the File Manager available in cPanel.