I had a similar problem but on MacOS and the reason was I had nvm installed. So running the commands found on most websites:
sudo npm cache clean -f
sudo npm install -g n
sudo n stable
didn’t work, node -v still displayed the old version.
What I did was to install node from nvm:
nvm install v0.12.6
This will build node from scratch on your machine, and then node -v will display the correct version. I think I could have still used the previous commands by using n and then do nvm use v0.12.6, but this is something to test. If you have nvm installed, just check the commands for nvm to upgrade node.