How to upgrade PostgreSQL from 9.5 to 9.6.1 without losing data?
After 9 months, I updated to PostgreSQL 10 from 9.6.5 with pg_upgrade:

1) Stop postgresql:

brew services stop postgresql

2) Initialize postgresql10 database:

initdb /usr/local/var/postgres10.0 -E utf8

3) Double check the newly created db:

ls /usr/local/Cellar/postgresql/

4) Run pq_upgrade to move data to new database:

pg_upgrade \
  -d /usr/local/var/postgres \
  -D /usr/local/var/postgres10.0 \
  -b /usr/local/Cellar/postgresql/9.6.5/bin/ \
  -B /usr/local/Cellar/postgresql/10.0/bin/ \
  -v


5) Rename old database:

mv /usr/local/var/postgres /usr/local/var/postgres9.6.5

6) Rename new database to postgres:

mv /usr/local/var/postgres10.0 /usr/local/var/postgres

7) Restart postgresql:

brew services start postgresql