If you are using a mac and you might run into issues in using npm update, try this recipe it worked for several computers that I tested it on.
Assuming that you installed it with Homebrew
brew updatebrew upgrade nodenpm install -g npm
After some use, permission on the local folder change, I have yet to understand the condition that generates that, however, if you run into this type of message
here is a quick fix for that too.
sudo chown -R $(whoami):admin /usr/local
Enjoy and I am hopeful that saves you some additional Google searches.