Very useful sites:
https://www.kostolansky.sk/posts/upgrading-to-postgresql-12/

https://stackoverflow.com/questions/12908205/whats-a-postgresql-cluster-and-how-do-i-create-one

Plus this:
Obsolete major version 9.5                                                                                                     ¦
  ¦                                                                                                                                ¦
  ¦ The PostgreSQL version 9.5 is obsolete, but the server or client packages are still installed. Please install the latest       ¦
  ¦ packages (postgresql-13 and postgresql-client-13) and upgrade the existing  clusters with pg_upgradecluster (see manpage).     ¦
  ¦                                                                                                                                ¦
  ¦ Please be aware that the installation of postgresql-13 will automatically create a default cluster 13/main. If you want to     ¦
  ¦ upgrade the 9.5/main cluster, you need to remove the already existing 13 cluster (pg_dropcluster --stop 13 main, see manpage   ¦
  ¦ for details).                                                                                                                  ¦
  ¦                                                                                                                                ¦
  ¦ The old server and client packages are no longer supported. After the existing clusters are upgraded, the postgresql-9.5 and   ¦
  ¦ postgresql-client-9.5 packages should be removed.                                                                              ¦
  ¦                                                                                                                                ¦
  ¦ Please see /usr/share/doc/postgresql-common/README.Debian.gz for details.