Free SSL certificates from Let's Encrypt certify are issued for a short 3 month term. I first enabled secure HTTPS pages around at start of 2016, and now it's time to renew SSL.
The following tutorial is written for Nginx, but should work for Apache as well (with minor modifications)
sudo service nginx stop
To trigger the renewal process for all installed domains, run this command:
sudo /opt/letsencrypt/letsencrypt-auto renew
sudo service nginx start
Now, check from your browser window your site, does SSL certificate validity has been extended for next 90 days,