How to install an old release of Helm on OSX using brew

Sometimes, your Kubernetes server needs an old version of helm installed.

On your host, you may have the latest helm release and you will have this kind of error message: Error: incompatible versions client[v2.9.0] server[v2.8.2]

You will need to downgrade the version installed on your mac if you don't want to upgrade the helm on your Kubernetes server but how?

Look on Github for the correct kubernetes-helm.rb file for the version you want (2.9.1 in my case): https://github.com/Homebrew/homebrew-core/search?q=kubernetes-helm&type=Commits
Click the commit hash
Click the "View" button to see the whole file
Click the "Raw" button
And copy the url: https://raw.githubusercontent.com/Homebrew/homebrew-core/78d64252f30a12b6f4b3ce29686ab5e262eea812/Formula/kubernetes-helm.rb

Now that you had the url for the correct kubernetes-helm.rb file, ran the following:

$ brew unlink kubernetes-helm
$ brew install https://github.com/Homebrew/homebrew-core/blob/560afd4e62f4b84113de9b27626325ded28abb87/Formula/kubernetes-helm.rb
$ brew switch kubernetes-helm 2.15.0