This assumes that you have Homebrew installed in first place and used it to install Node. To manage multiple Node versions on your development server, first search for your desired pacakage
brew search node
Pick a version of your choice and run
brew install node@6
Remember you can install more than one Node package but can't have them available at the same time.
If you have latest/generic node already installed, unlink it first
brew unlink node
and link to your desired version
brew link node@6 --force