Upgrading is the same as installing you just curl the latest version and apt-get install. For example to upgrade to version 7 you would run:
curl -sL https://deb.nodesource.com/setup_7.x | sudo -E bash -
apt-get install -y nodejs
For the full install guide see Installing basic Node.js dev env on Windows 10 bash
Full list of available versions from nodesource and update the setup_7.x as appropriate.
Check out the nodejs site to find out which versions is best for you.