http://collectiveidea.com/blog/archives/2016/01/08/postgresql95-upgrade-with-homebrew/
Upgrade Postgres (9.5):
brew update && brew upgrade postgres
Then let’s stop the old installation (but don’t load up the new one yet).
launchctl unload ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist
Next, we initialize an empty database:
initdb /usr/local/var/postgres9.5 -E utf8
Then we do the upgrade. First, find out exactly which version you had before:
ls /usr/local/Cellar/postgresql/
Which gives you output something like this: 9.4.5_2 9.5.0. If you have more than two, that’s ok, just get the most recent one before 9.5. In my case, that was 9.4.5_2. Remember that version for the next step.
Run pg_upgrade and put the version number we remembered above in the 4th line here.
pg_upgrade \
-d /usr/local/var/postgres \
-D /usr/local/var/postgres9.5 \
-b /usr/local/Cellar/postgresql/9.4.5_2/bin/ \
-B /usr/local/Cellar/postgresql/9.5.0/bin/ \
-v
Next, move the new directory to where postgres expects it to be.
mv /usr/local/var/postgres /usr/local/var/postgres-9.4.5_2
mv /usr/local/var/postgres9.5 /usr/local/var/postgres
You could alternately move the old directory to the trash. I don’t like to delete it completely until we’ve made sure our upgrade works.
Finally, start up PostgreSQL again:
launchctl load ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist
Test out your application(s) (for me, I fired up a rails console and pulled up a record). You can clean up the directory and run brew cleanup if it all worked.