You can install the latest version of Ionic and Cordova CLI using a single command

$ npm install ionic cordova -g

Still, you may get a notification message saying

“Please install your Cordova CLI to version >=4.2.0 `npm install -g cordova`”

Or, incorrect version numbers for ionic and cordova even after installing their latest versions globally.

In these situations, you can try the following options

i) Clear npm cache
$ npm cache clean -f

ii) Update npm globally
$ npm install npm -g

If the problem persists, try uninstalling both ionic and cordova and then re-install it again to see if this fixes the issue.