BlueBell's answer is right, you can do it by:

npm uninstall cordova ionic

Are you planning to re-install it? If you feel something's wrong which is causing problems then you should update npm and clean npm's cache.

npm cache clean -f
npm install npm -g

If problems still persist, I'd suggest re-install of NPM and Node.

npm uninstall node
apt-get purge npm
apt-get install npm
npm install node -g

Let me know if you face issues in the process.

To uninstall Ionic and Cordova:

sudo npm uninstall -g cordova
sudo npm uninstall -g ionic

To install:

sudo npm install -g cordova

Command to remove Cordova and ionic

• For Window system

  • npm uninstall -g ionic
  • npm uninstall -g cordova

• For Mac system

  • sudo npm uninstall -g ionic
  • sudo npm uninstall -g cordova

• For install cordova and ionic

  • npm install -g cordova
  • npm install -g ionic

Note:

• If you want to install in MAC System use before npm use sudo only.
• And plan to install specific version of ionic and cordova then use @(version no.).

eg.

sudo npm install -g cordova@6.0.0

sudo npm install -g ionic@2.1.18