# used to find an existing elm package installed and delete it, this command works to remove a 0.18 or 0.19 elm and reinstall one of the selected ones
sudo find . -name "elm*" -exec rm -rf {} \;

# install elm@0.19
sudo npm install -g elm --unsafe-perm=true --allow-root

# or

# install elm@0.18
sudo npm install -g elm@0.18.0 --unsafe-perm=true --allow-root