phangorn is a package for R. To use it, you first need to download and install R. RStudio provides a nice user interface for R.
You can install the latest release version from CRAN
install.packages("phangorn")
or install the latest development version using the install_github function in
the devtools package from github.
library(devtools)
install_github("KlausVigo/phangorn")
For devtools to work on windows you need addionally to have installed Rtools and on mac you need Xcode to compile some C code. Once these programs are installed you can install phangorn with the commands above. The development version of phangorn depends on the latest deveolpment version from the ape package. Instructions to for download and installation of ape can be found here.