How can I downgrade my ionic cli to the previous version, I had?

First thing first, uninstall the current version:

    npm uninstall -g ionic

then, install the version you want :

   npm install -g ionic@"the version for example @3.20.0"

You just need to do like this:

npm install -g ionic@3.1.0

Changelog

Sometimes it throws error while overwriting the existing files , so add --force

Ex, If you want to downgrade to version 3.20, run: npm i -g ionic@3.20 --force