It's easy:

1) Change your BASE_URL in the /config/site.php file so that 'http://example.com' becomes 'https://example.com'. You are adding the 's'.

2) Alter your .htaccess file to force SSL by adding these lines below this line 'RewriteBase /':

RewriteCond %{HTTPS} off

RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R,L]

That's it! The .htaccess change will force https and the site.php change will let concrete5 know that https should be in the URL.