If the setup is correct for Lets encrlypt, then the site should be automatically updating its certificate before it expires in 30 days.  One of my site did not update.

 

I looked at the config file for this partiucular site and noticed installer and authenticator was not set up for nginx.  Hopefully next time it will update.

 

In the meantime,  here is the command I can force update the site certificate

 

certbot --force-renewal -d sitename1, sitename2,....