If you've installed a couple versions of a program via brew and you'd like
to switch from the currently linked version to the other installed version,
you can use the switch command. For instance, if you are on version
1.8.2 of phantomjs and you'd like to switch to 1.9.0, you can simply
invoke:
$ brew switch phantomjs 1.9.0
More generically:
$ brew switch <formula> <version>