Got an SSL certificate for your domain? Want to use HTTPS instead of HTTP for secure data transfer and better performance? Really Simple SSL makes it a bit easier to make the switch, taking care of redirecting requests to HTTPS, and ensuring that green padlock in the address bar.
Extract from WordPress.org
Really Simple SSL automatically detects your settings and configures your website to run over https. To keep it lightweight, the options are kept to a minimum. The entire site will move to SSL.
https://wordpress.org/plugins/really-simple-ssl/