A) First create a backup of all the databases for that (You can continue from B if you dont need a backup)

1. Log in as postgres user

sudo su postgres

2. Create a backup .sql file for all the data you have in all the databases

pg_dumpall > backup.sql

B) Upgrade to PostgreSQL12

1. update packages and install postgres 12

sudo apt-get update
sudo apt-get install postgresql-12 postgresql-server-dev-12

2. Stop the postgresql service

sudo systemctl stop postgresql.service

3. migrate the data

/usr/lib/postgresql/12/bin/pg_upgrade \
--old-datadir=/var/lib/postgresql/10/main \
--new-datadir=/var/lib/postgresql/12/main \
--old-bindir=/usr/lib/postgresql/10/bin \
--new-bindir=/usr/lib/postgresql/12/bin \
--old-options '-c config_file=/etc/postgresql/10/main/postgresql.conf' \
--new-options '-c config_file=/etc/postgresql/12/main/postgresql.conf'

4. Switch to regular user

exit

5. Swap the ports the old and new postgres versions.

#change port to 5432
sudo vim /etc/postgresql/12/main/postgresql.conf
#change port to 5433
sudo vim /etc/postgresql/10/main/postgresql.conf

6. Start the postgresql service

sudo systemctl start postgresql.service

7. Log in as postgres user

sudo su postgres

8. Check your new postgres version

psql -c "SELECT version();"

8. Run the generated new cluster script

./analyze_new_cluster.sh

9. Return as a normal(default user) user and cleanup up the old version's mess

#uninstalls postgres packages     
sudo apt-get remove postgresql-10 postgresql-server-dev-10
#removes the old postgresql directory
sudo rm -rf /etc/postgresql/10/
#login as postgres user
sudo su postgres
#delete the old cluster data
./delete_old_cluster.sh

10. Congrads! Your postgresql version is now upgraded, If everything works well in B, we dont have to apply the backup as we have already migrated the data from the older version to the newer version, the backup is just in case if anything goes wrong.

NOTE: Change the postgresql.conf and pg_hba.conf as per your requirement

PS: Feel free to comment your issues, suggestions or anyother modifications you would like to suggest

1. Backup the database
   

   psql --version sudo -u postgres psql pg_dumpall > alldbs.sql
   

   (this command will backup all databases from the postgresql db)

Then exit from postgres user, and:

2. Inside a terminal run these commands:

   sudo systemctl stop postgres
   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 10 main
   sudo pg_dropcluster 10 main --stop
   

3. restart the postgresql service:

   sudo systemctl restart postgresql
   

4. login to the postgres:

   su - postgres
   

5. to check the version:

   psql --version
   

I have done using the above steps and I could update the DBand restore all data.

Postgresql Upgrade using pg_upgrade

Step - 1 (Find the locale language and encoding of source Postgresql server)

• Log in to Postgresql database as postgres user
• Execute the query SHOW LC_COLLATE;

Step - 2 ( Stop the existing/source Postgresql server)

• Find the running Postgresql service using $ sudo systemctl list-units | grep postgres
• Stop the service $ sudo service postgresql-<source-version>.service stop

Step - 3 (Install target Postgresql server)

• Configure repos and install (lot of tutorials available in google)

Step - 4 (Update the locale language and encoding)

$ /usr/pgsql-<target-version>/bin/initdb -D /var/lib/pgsql/<target-version>/data --locale=<add-your-encoding>

Step - 5 (Check the source to target upgrade has any potential issues)

• Run the command as postgres user, $sudo su postgres

  $  /usr/pgsql-<target-version>/bin/pg_upgrade --old-bindir /usr/pgsql-<source-version>/bin --new-bindir /usr/pgsql-<target-version>/bin --old-datadir /var/lib/pgsql/<source-version>/data --new-datadir /var/lib/pgsql/12/data --link --jobs=2 --check

If it's ok we can move to the next step, If not fix the issues before proceeding to the next step Expected output: Clusters are compatible

Step - 6 (Upgrade the source to target Postgresql version using the link option)

• link option is pretty faster than the migration
• Run the command as postgres user, $sudo su postgres

    $ /usr/pgsql-<target-version>/bin/pg_upgrade --old-bindir /usr/pgsql-<source-version>/bin --new-bindir /usr/pgsql-<target-version>/bin --old-datadir /var/lib/pgsql/<source-version>/data --new-datadir /var/lib/pgsql/<target-version>/data --link

Expected output: Upgrade Complete

Thanks. ENJOY !!!