brew update
Error: The /usr/local directory is not writable.
Even if this directory was writable when you installed Homebrew, other
software may change permissions on this directory. For example, upgrading
to OS X El Capitan has been known to do this. Some versions of the
"InstantOn" component of Airfoil or running Cocktail cleanup/optimizations
are known to do this as well.

You should probably change the ownership and permissions of /usr/local
back to your user account.

sudo chown -R setup.admin:admin /usr/local/
chown: /usr/local: Operation not permitted