**TL;DR**

Create a backup:


```bash
sudo -Hiu postgres pg_dumpall > mybackup.sql
```

Install Postgres 11, and then:

```bash
sudo pg_dropcluster 11 main --stop
sudo pg_upgradecluster 9.x main
sudo pg_dropcluster 9.x main

(x is a sub-version number)
```

---
Install PostgreSQL 11:

```bash
# install **wget** if not already installed:
sudo apt install -y wget

# import the repository signing key:
wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -

# add repository contents to your system:
RELEASE=$(lsb_release -cs)
echo "deb http://apt.postgresql.org/pub/repos/apt/ ${RELEASE}"-pgdg main | sudo tee  /etc/apt/sources.list.d/pgdg.list

# install and launch the postgresql service:
sudo apt update
sudo apt -y install postgresql-11
```

Use `dpkg -l | grep postgresql` to check which versions of postgres are installed:

```
ii  postgresql-11                                 object-relational SQL database, version 11 server
ii  postgresql-9.6                                object-relational SQL database, version 9.6 server
ii  postgresql-client-11                          front-end programs for PostgreSQL 11
ii  postgresql-client-9.6                         front-end programs for PostgreSQL 9.6
ii  postgresql-client-common                      manager for multiple PostgreSQL client versions
ii  postgresql-common                             PostgreSQL database-cluster manager
ii  postgresql-contrib-9.6                        additional facilities for PostgreSQL

```

In my case, I will be performing the upgrade from 9.6 to 11 version, so at the next steps, I will replace the 9.x version with the actual 9.6 version. 

**Note:** guide can be also used to upgrade PostgreSQL 10 version. Upgrade steps fully the same.

Run `pg_lsclusters`, your 9.6 and 11 main clusters should be "online".

```
pg_lsclusters 
Ver Cluster Port Status Owner    Data directory               Log file
9.6 main    5432 online postgres /var/lib/postgresql/9.6/main /var/log/postgresql/postgresql-9.6-main.log
 11 main    5433 online postgres /var/lib/postgresql/11/main  /var/log/postgresql/postgresql-11-main.log
```

There already is a cluster "main" for 11 (since this is created by default on package installation).
This is done so that a fresh installation works out of the box without the need to create a cluster first,
but of course it clashes when you try to upgrade 9.6/main when 11/main also exists.
The recommended procedure is to remove the 11 cluster with `pg_dropcluster` and then upgrade the 9.6/main with `pg_upgradecluster`.

Stop the 11 cluster and drop it.

```bash
sudo pg_dropcluster 11 main --stop
```

Upgrade the 9.6 cluster to the latest version.

```bash
sudo pg_upgradecluster 9.6 main
```

Your 9.6 cluster should now be "down".

```
pg_lsclusters 
Ver Cluster  Port Status Owner    Data directory               Log file
9.6 main     5433 down   postgres /var/lib/postgresql/9.6/main /var/log/postgresql/postgresql-9.6-main.log
11  main     5432 online postgres /var/lib/postgresql/11/main /var/log/postgresql/postgresql-11-main.log
```

Check that the upgraded cluster works, then remove the 9.6 cluster.

```bash
sudo pg_dropcluster 9.6 main
```

Lastly, please make sure that in the postgresql.conf for new cluster **cluster_name** parameter is equals to '11/main', otherwise replace the existing value with '11/main':

```
sudo nano /etc/postgresql/11/main/postgresql.conf
```

change `cluster_name = '9.6/main'` to `cluster_name = '11/main'`.