Thanks Matt! “sudo apt upgrade” did the trick. I have certbot 0.28.0 now.
The email “Action required: Let’s Encrypt certificate renewals” that I received had this in it:
“If you’re a Certbot user, you can find more information here:
How to stop using TLS-SNI-01 with Certbot”
When you follow the link you get these instructions:
" If the version is less than 0.28, you need to upgrade your Certbot. Visit https://certbot.eff.org/ and follow the instructions for your webserver and OS."
When you follow that link you get the commands to install Certbot. But they do not upgrade the version.
Perhaps another email should be sent with more clear instructions on how to upgrade an existing Certbot installation.