Community in the Cloud customers can submit a support ticket to request SSL/HTTPS be enabled for your community.
You will need to obtain an SSL certificate and install it on your server before following these instructions
To enable SSL/HTTPS for your community, please edit your conf_global.php file, changing 'http' to 'https' in your URL, then save.
$INFO['base_url'] = 'http://www.yourdomain.com';
$INFO['base_url'] = 'https://www.yourdomain.com';
(it's possible that this parameter may be called board_url in your configuration)
Then log into the AdminCP > Support > 'Clear System Caches'. This clears out your cache, and will allow all items to then be linked from HTTPS.
Please also keep in mind that all advertisements or any code you may have added to the site or theme must be served from https or the cert will show an error for your users.