DigitalOcean has a 1-Click app for Ghost created by the Ghost team. As part of the app, the system requests a new Letsencrypt SSL certificate.
These certificates are renewed automatically every three months via cron.
However, occationally the cron job fails and a new certificate needs to be created and installed for Ghost. The process is fairly simple
First, run the command sudo /etc/letsencrypt"/acme.sh --upgrade --home "/etc/letsencrypt" which updates letsencrypt on the system.
Second, run the command sudo /etc/letsencrypt"/acme.sh --cron --home "/etc/letsencrypt" > /dev/null which then updates the certificate from Letsencrypt and installs it in the correct place for Nginx and Ghost.
Wait 30 seconds and then refresh the Ghost blog and you should have a fresh Letsencrypt SSL certificate