I have received this message and don't know how to do the renovation.
Your certificate (or certificates) for the names listed below will expire in 20 days (on 23 Jan 21 22:20 +0000). Please make sure to renew your certificate before then, or visitors to your website will encounter errors.
We recommend renewing certificates automatically when they have a third of their
total lifetime left. For Let's Encrypt's current 90-day certificates, that means
renewing 30 days before expiration. See
https://letsencrypt.org/docs/integration-guide/ for details.
What I have to do in order to make this renovation of this certificates real?