If you want a run-down of the 1.3 changes and the design decisions behidn those changes, check out the LonestarElixir Phoenix 1.3 keynote: https://www.youtube.com/watch?v=tMO28ar0lW8
To use the new phx.new project generator, you can install the archive with the following command:
$ mix archive.install https://github.com/phoenixframework/archives/raw/master/phx_new.ez
Phoenix v1.3.0 is a backwards compatible release with v1.2.x. To upgrade your existing 1.2.x project, simply bump your phoenix dependency in mix.exs: