I want to downgrade my version of node. However, I don't remember how I installed it. Apparently I didn't use Homebrew (an OS X package manager). In any case, how do I completely uninstall given that I don't know how I installed it?
How you installed Node determines how straightforward the process of uninstallation will be. If you installed it from MacPorts,
sudo port uninstall nodejs should do the trick; if you installed it from source or from a tarball, that makes things a bit trickier, as there is usually no "make uninstall" or similar command available.
On the other hand, it may not be necessary to uninstall the version of Node you already have in place; packages installed via a properly configured Homebrew will override system-level installations. Consider simply installing the version you want through Homebrew. Also, if the existing Node is installed at the system level, then downloading the tarball of the version you want to use, and installing it in the same fashion, will probably (not certainly) overwrite the currently installed version and leave you with a working install of the version you want.