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So you probably should have given one npm install -g ionic
right? Doing this npm will always fetch the latest officially available updated version, in case there 4.12.0 for you to install a previous version (downgrade, otherwise upgrade) can follow the following steps:
Step 1 - Uninstall version 4.12.0 previously installed with the command
npm uninstall -g ionic
Step 2 - Install exactly the version you prefer
npm install -g [email protected]
This "forces" npm to download and install exactly this version.