This is the new best way to upgrade npm on Windows.
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.
npm-windows-upgrade -p -v latest.