It is not something you may want to always keep updated.

Check the latest version of NPM.
https://www.npmjs.com/

At the time of this post, the latest version is 3.5.0

To check your current npm version type: $ npm --version

3.3.12 is my npm version readout.

To upgrade my current version to the latest version, enter the following command:
npm install npm@latest -g