In order to find the npm latest stable version, you need to visit the npm documentation web page and check the footer.

Earlier it used to mention the version number in footer but now it provides a link to the GitHub hosted repository https://github.com/npm/cli/releases/latest; so, you can directly go to this link and check the latest stable npm version available.

To upgrade to the npm latest stable version available, run the following command command line interface[CLI]:

npm install npm@latest -g

or 

npm i npm@latest -g