First type which node && which npm if you get anything other than a blank line you already have node installed and need to remove it. If you used the node installer from their website use these commands:
sudo rm -rf $(which node)
sudo rm -rf $(which npm)
sudo rm -rf ~/.node
sudo rm -rf ~/.npm
Now if you type which node && which npm you should have a blank line.
These instructions will help you install the latest version of node in a way the prevents you from needing root to install global packages. You will need curl, python v2.x a C compiler(gcc or clang) and make installed. These can be obtained through whatever package manager your operating system uses (homebrew on Mac, apt-get on Ubuntu, etc).
curl -O https://nodejs.org/dist/v4.4.3/node-v4.4.3.tar.gz
tar -vxzf node-v4.4.3.tar.gz
cd node-v4.4.3
./configure --prefix=$HOME/.node
make && make install
These commands will download the latest stable version of node (currently 4.4.3) configure it to install into a .node folder in your home directory and will then compile it from source. Next you need to tell your shell to look for the node command in $HOME/.node/bin
On Linux:
echo "export PATH=$HOME/.node/bin:$PATH" >> ~/.bashrc
echo "export NODE_PATH=$HOME/.node/lib/node_modules" >> ~/.bashrc
source ~/.bashrc
On Mac:
echo "export PATH=$HOME/.node/bin:$PATH" >> ~/.bash_profile
echo "export NODE_PATH=$HOME/.node/lib/node_modules" >> ~/.bash_profile
source ~/.bash_profile
If you now enter the command node --version you should see v4.4.3