Initially, I tried a brew install postgresql over the top of the existing version that was installed via non-Homebrew means. When it was complete, psql --version was still showing 8.4.x. I found it is best to uninstall any old PostgreSQL versions first. Of course,back up your data with pgAdmin or pg_dump.
First, uninstall any Homebrew versions. The --force option makes it uninstall all versions.
brew rm postgresql --force
Second, uninstall any versions installed by other means.
If you installed using EnterpriseDB, you likely have an uninstall-postgresql.app file in your install directory. Double-click that to uninstall their Postgres install.
I went the manual route. Change paths accordingly for your version.
The Homebrew formula does not create a “postgres” user, as was common up to this point. Out of the box, you authenticate against the DB using the same user account under which you ran brew install. It’s perfectly acceptable to leave it that way on your Dev machine. Don’t forget to change that if this is a Production environment.
Install Instrumentation so pgAdmin doesn’t yell at you. Run this as the same user under which you installed Postgres.