**TL;DR**

Install Postgres 9.6, and then:

```bash
sudo pg_dropcluster 9.6 main --stop
sudo pg_upgradecluster 9.5 main
sudo pg_dropcluster 9.5 main
```

---

Specifically for using the WAL-E enabled Docker image, make sure WAL-E is turned off:

```
sudo pkill backupcron
```

---
Install PostgreSQL:

```
sudo echo 'deb http://apt.postgresql.org/pub/repos/apt/ xenial-pgdg main' > /etc/apt/sources.list.d/pgdg.list
wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | \
sudo apt-key add -
  
sudo apt update

sudo apt install postgresql-9.6 postgresql-client-9.6 postgresql-contrib-9.6
```

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

```
i   postgresql                                                        - object-relational SQL database (supported version)
i A postgresql-9.5                                                    - object-relational SQL database, version 9.5 server
i A postgresql-9.6                                                    - object-relational SQL database, version 9.6 server
i A postgresql-client-9.5                                             - front-end programs for PostgreSQL 9.5
i A postgresql-client-9.6                                             - front-end programs for PostgreSQL 9.6
i A postgresql-contrib-9.5                                            - additional facilities for PostgreSQL
i A postgresql-contrib-9.6                                            - additional facilities for PostgreSQL
```

Run `pg_lsclusters`, your 9.5 and 9.6 main clusters should be "online".

```
pg_lsclusters 
Ver Cluster Port Status Owner    Data directory               Log file
9.5 main    5432 online postgres /var/lib/postgresql/9.3/main /var/log/postgresql/postgresql-9.5-main.log
9.6 main    5433 online postgres /var/lib/postgresql/9.5/main /var/log/postgresql/postgresql-9.6-main.log
```

There already is a cluster "main" for 9.6 (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 9.5/main when 9.6/main also exists.
The recommended procedure is to remove the 9.6 cluster with `pg_dropcluster` and then upgrade with `pg_upgradecluster`.

Stop the 9.6 cluster and drop it.

```bash
sudo pg_dropcluster 9.6 main --stop
```

Stop and Upgrade the 9.5 cluster to the latest version.

```bash
sudo service postgresql stop
sudo pg_upgradecluster 9.5 main
```

Note that the upgrade can take a long time for large databases. Eg a 15GB database can take hours (from personal experience). In this case you might want to run the pg_upgrade command manually with the `-k -j8` switches. `-k` creates symlinks to the original database and works directly on that, rather than copying it over. SO THIS IS A DESTRUCTIVE OPERATION - ONLY DO IF YOU HAVE BACKUPS. The `-j` switch specifies how many CPU cores to use in the upgrade. Use 8 if you have an 8-core CPU.

```bash
sudo service postgresql stop
sudo -H -u postgres /usr/lib/postgresql/9.6/bin/pg_upgrade \
   -b /usr/lib/postgresql/9.5/bin \
   -B /usr/lib/postgresql/9.6/bin \
   -d /var/lib/postgresql/9.5/main \
   -D /var/lib/postgresql/9.6/main \
   -o ' -c config_file=/etc/postgresql/9.5/main/postgresql.conf' \
   -O ' -c config_file=/etc/postgresql/9.6/main/postgresql.conf' \
   -k -j8
```

Your 9.5 cluster should now be "down".

```
pg_lsclusters 
Ver Cluster Port Status Owner    Data directory               Log file
9.5 main    5433 down   postgres /var/lib/postgresql/9.5/main /var/log/postgresql/postgresql-9.5-main.log
9.6 main    5432 down postgres /var/lib/postgresql/9.6/main /var/log/postgresql/postgresql-9.6-main.log
```

Check that the upgraded cluster works, then remove the 9.5 cluster.

```bash
sudo service postgresql start
sudo pg_dropcluster 9.5 main
```

If seeing file permission errors upon startup:
```
 * The PostgreSQL server failed to start. Please check the log output:
2017-03-14 04:06:51 UTC FATAL:  private key file "/var/lib/postgresql/ssl/ssl-cert-snakeoil.key" has group or world access
2017-03-14 04:06:51 UTC DETAIL:  File must have permissions u=rw (0600) or less if owned by the database user, or permissions u=rw,g=r (0640) or less if owned by root.
2017-03-14 04:06:51 UTC LOG:  database system is shut down
   ...fail!
```

Then simply fix the permissions:
```
sudo chmod 600 /var/lib/postgresql/ssl/*.key
```

If using WAL-E, push a new base backup:

```
sudo -u postgres envdir /etc/wal-e.d/env wal-e backup-push
```


Lastly update config (if required):

```
sudo vi /etc/postgresql/9.6/main/postgresql.conf
```

Change `cluster_name = '9.5/main'` to `cluster_name = '9.6/main'`.