The new best way to upgrade NPM on Windows:

https://github.com/felixrieseberg/npm-windows-upgrade

Run PowerShell as Administrator

Set-ExecutionPolicy Unrestricted -Scope CurrentUser -Force
npm install -g npm-windows-upgrade
npm-windows-upgrade

Note: Do not run npm i -g npm. Instead use npm-windows-upgrade to update npm going forward. Also if you run the NodeJS installer, it will replace the node version.

• Upgrades npm in-place, where node installed it.
• Easy updating, update the to the latest by running npm-windows-upgrade -p -v latest.
• Does not modify the default path.
• Does not change the default global package location.
• Allows easy upgrades and downgrades.
• Officially recommended by the NPM team.

Source: http://stackoverflow.com/questions/18412129/how-do-i-update-node-and-npm-on-windows