edit your existing homebrew formula eg.: $rafael: brew edit python3
Change the url version to your desired version, eg: url "https://www.python.org/ftp/python/3.6.1/Python-3.6.1.tar.xz" to url "https://www.python.org/ftp/python/3.4.1/Python-3.4.1.tar.xz"
save with another name, eg: