You need to follow the Windows upgrade instructions ( https://docs.npmjs.com/try-the-latest-stable-version-of-npm )
tl;dr – npm -g install npm
does work, but the old version of npm is still in your PATH.
To fix this, do one of these:
Option 2: remove both of
C:\Program Files (x86)\nodejs\npm
C:\Program Files (x86)\nodejs\npm.cmd
Or
Option 3: Open cmd.exe as administrator, navigate to C:\Program Files (x86)\nodejs and
then run the installation without -g:
npm install npm@latest
*There is an npm package that automate this Option 3:
npm install -g npm-win-upgrade