This works on my mac.
Based on docs https://docs.npmjs.com/troubleshooting/try-the-latest-stable-version-of-npm :
npm install -g npm@latest
There is a note stated on the docs that depends on your installation method, you might addd some sudo.
Upgrading on *nix (OSX, Linux, etc.)
(You may need to prefix these commands with sudo, especially on Linux,
or OS X if you installed Node using its default installer.)