Error: npm is known not to run on Node.js V4.2.6

First, Uninstall completely nodejs and npm.

sudo apt remove nodejs npm

Then, reinstall it over the link below:

curl -sL https://deb.nodesource.com/setup_6.x | sudo -E bash -
sudo apt-get install -y nodejs

Refer: https://nodejs.org/en/download/package-manager/#debian-and-ubuntu-based-linux-distributions