So follow these steps to upgrade node:
sudo npm cache clean -f
sudo npm install -g n
sudo n stable
sudo ln -sf /usr/local/n/versions/node/5.4.1/bin/node /usr/bin/node
(the bold text/version should be the one installed in during above step.)
ie if 8.1.1 is installed then do sudo ln -sf /usr/local/n/versions/node/8.1.1/bin/node /usr/bin/node
node –v
(Should show updated version now)