TL;DR
sudo apt-get install -y postgresql-12 postgresql-server-dev-12 postgresql-contrib-12 libpq-dev postgresql-12-hypopg
sudo pg_dropcluster 12 main --stop
sudo pg_upgradecluster 11 main
sudo pg_dropcluster 11 main
TL;DR
sudo apt-get install -y postgresql-12 postgresql-server-dev-12 postgresql-contrib-12 libpq-dev postgresql-12-hypopg
sudo pg_dropcluster 12 main --stop
sudo pg_upgradecluster 11 main
sudo pg_dropcluster 11 main