TL;DR
Install Postgres 11, and then:
sudo pg_dropcluster 11 main --stop
sudo pg_upgradecluster 9.5 main
sudo pg_dropcluster 9.5 main
TL;DR
Install Postgres 11, and then:
sudo pg_dropcluster 11 main --stop
sudo pg_upgradecluster 9.5 main
sudo pg_dropcluster 9.5 main