Question: I have a few kegs of the same package in `/usr/local/Cellar/libfoo` like `/usr/local/Cellar/libfoo/1.0.1`, `/usr/local/Cellar/libfoo/HEAD` and `/usr/local/Cellar/libfoo/mycopy`

How can I brew link to a specific version?

Answer: 
If you have installed, for example, php 5.4 it could be switched in the following way to php 5.5:

```
$ php --version
PHP 5.4.32 (cli) (built: Aug 26 2014 15:14:01) 
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2014 Zend Technologies

$ brew unlink php54

$ brew switch php55 5.5.16

$ php --version
PHP 5.5.16 (cli) (built: Sep  9 2014 14:27:18) 
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2014 Zend Technologies

```
if @simon's answer is not working in some of the mac's please follow the below process.

If you have already installed swiftgen using the following commands:

$ `brew update`
$ `brew install swiftgen`

then follow the steps below in order to run swiftgen with older version.

Step 1: `brew uninstall swiftgen`
Step 2: Navigate to: <https://github.com/SwiftGen/SwiftGen/releases>
and download the swiftgen with version: swiftgen-4.2.0.zip.

Unzip the package in any of the directories.

Step 3:
Execute the following in a terminal:

```
$ mkdir -p ~/dependencies/swiftgen
$ cp -R ~/<your_directory_name>/swiftgen-4.2.0/ ~/dependencies/swiftgen
$ cd /usr/local/bin
$ ln -s ~/dependencies/swiftgen/bin/swiftgen swiftgen
$ mkdir ~/Library/Application\ Support/SwiftGen
$ ln -s ~/dependencies/swiftgen/templates/ ~/Library/Application\ Support/SwiftGen/

$ swiftgen --version

```

You should get: SwiftGen v0.0 (Stencil v0.8.0, StencilSwiftKit v1.0.0, SwiftGenKit v1.0.1)

[![enter image description here](https://i.stack.imgur.com/YKnK2.png)](https://i.stack.imgur.com/YKnK2.png)