To check the installed version:

$ mix phx.new -v
Phoenix installer v1.7.11

To update, you can do this:

$ mix archive.install hex phx_new
Resolving Hex dependencies...
Resolution completed in 0.041s
New:
  phx_new 1.7.14
* Getting phx_new (Hex package)
All dependencies are up to date
Compiling 11 files (.ex)
Generated phx_new app
Generated archive "phx_new-1.7.14.ez" with MIX_ENV=prod
Found existing entry: /Users/me/.mix/archives/phx_new-1.7.11
Are you sure you want to replace it with "phx_new-1.7.14.ez"? [Yn] y
* creating /Users/me/.mix/archives/phx_new-1.7.14