You can manually renew expiring certificates in the Dashboard → System & Settings → HTTPS → HTTPS certificates page of your concrete5 installation (next you'll need to tell Apache/nginx to reload the new HTTPS certificate files).

By the way, for security reasons the Let's Encrypt certificates have a short validity (90 days at the time of writing this), so this could become tedious.

You can automate the certificate renewals by scheduling a shell script to be run on your webserver.

Indeed, this package comes with the command le:renew command that can be used in a terminal instance.

For example, here's how you can schedule certificate renewal if your web server have Apache and Ubuntu:

1. Create a file in the daily scheduled tasks (for instance with sudo nano /etc/cron.daily/c5-letsencrypt-renew)
2. Enter this code in that new file:

   #!/bin/sh
   
    /path/to/webroot/concrete/bin/concrete5 le:renew && service apache reload 

3. That's all, you can go out for a beer: concrete5 will handle your https certificates automatically ;)