Your NPM version is tied to your NodeJS version. As far as I can tell you can only have one NPM version per Node version. Using something like nodenv or, my favorite, asdf, you can define your node version per folder.
Per Node version (e.g. per folder) you globally install the version of NPM that you want to use.
$ npm install -g [email protected]
Edit: You can also now specify NPM version in your package.json “engines” key, though it takes a little more work to enforce that declaration.