How to upgrade Node.js?

I'm getting started with Node.js and Express. So far I've created an Express project in PyCharm and I see it's set to use a version of Node.js that is installed in a PyCharm subdirectory. I was wondering…

(1) How do I upgrade Node.js? Is it possible to install newer versions of Node.js such that it's sandboxed adnd doesn't interfere with the rest of my system?

(2) Why is it that when I click “Terminal” and attempt to execute either node or npm I get “command not found”? However when I click “run” my application is able to execute.