Points to Remember

If you are installing node for the first time then, use the NodeSource PPA and first choose the version of node you want to install.

Following are the sources for the node versions

for Node.js v4

curl -sL https://deb.nodesource.com/setup_4.x | sudo -E bash -

for Node.js v5

curl -sL https://deb.nodesource.com/setup_5.x | sudo -E bash -

for Node.js v6

curl -sL https://deb.nodesource.com/setup_6.x | sudo -E bash -

for Node.js v7

curl -sL https://deb.nodesource.com/setup_7.x | sudo -E bash -

Once this is done, then update your repositories by command

sudo apt-get update

Now, finally install NodeJs and Npm using the following command

sudo apt-get install nodejs
sudo apt-get install npm

To check the version of node, try command node -v and npm version by command npm -v

Upgrading NodeJs and Npm

If you already have node and npm install and you want to upgrade the version then you should first run the following command to remove them.

sudo apt-get purge nodejs npm

This will remove nodejs and npm, now you can follow the steps mentioned above to install fresh copy of node and npm.