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Add the PostgreSQL package repository on your linux machine

echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main 11" | tee /etc/apt/sources.list.d/pgsql.list

Add the GPG key of the PostgreSQL package repository

wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add -

Update sources and install new postgres version

apt update && postgresql-12 postgresql-server-dev-12

Stop all postgres instances running

systemctl stop postgresql.service

Run the upgrade

/usr/lib/postgresql/12/bin/pg_upgrade -v -d /etc/postgresql/9.6/main -D /etc/postgresql/12/main/ -b /usr/lib/postgresql/9.6/bin/ -B /usr/lib/postgresql/12/bin/

Here change port to 5432

vim /etc/postgresql/12/main/postgresql.conf

 Start postgres service

systemctl start postgresql.service

If needed remove old postgres

Check installed postgres

apt list --installed | grep postgresql

Remove old version from folder

rm -rf /etc/postgresql/11/

Disable postgres version from run

/etc/postgresql/9.1/main/start.conf

Change from auto to disable. See here