RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://yourdomain.com/$1 [R,L]

 

If your site is in a subfolder, use this code:

RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteCond %{REQUEST_URI} folder
RewriteRule ^(.*)$ https://yourdomain.com/folder/$1 [R,L]

Simply replace yourdomain.com with your actual domain name.