This is the easiest part with no coding required. To make the site HTTPS, you can make use of Let’s Encrypt or Cloudflare to have an SSL Certificate installed for your domain. You can also take help from Domain registrar for this, as these domain hosting services also provide SSL certificate installation in a few clicks. 

Making site HTTPS is important because this keeps the site connected with secure connection and also enables the website to make use of future technologies like PWA’s service workers, that will in turn can make the site offline ready, enabling web push notifications for the subscribers, better loading and performance by saving time to load same assets loading which are cached by PWA.

You can read more about Why HTTPS matters.