How can I update NPM to the next version?

How do I update npm?

npm install -g npm

Please note that this command will remove your current version of npm. Make sure to use sudo npm install -g npm if on a Mac.
You can also update all outdated local packages by doing npm update without any arguments, or global packages by doing npm update -g.
Occasionally, the version of npm will progress such that the current version cannot be properly installed with the version that you have installed already. (Consider, if there is ever a bug in the update command.) In those cases, you can do this:

curl https://www.npmjs.com/install.sh | sh