how to update node.js 6.x to 8.x in windows operating sysytem using CLI

Just download the latest version of nodejs and install it without uninstalling the previous version, the older version will be rewritten with the newer one. you wouldn't be having any compatibility issues.

First, ensure that you can execute scripts on your system by running the following command from an elevated PowerShell. To run PowerShell as Administrator, click Start, search for PowerShell, right-click PowerShell and select Run as Administrator.

Set-ExecutionPolicy Unrestricted -Scope CurrentUser -Force

npm install --global --production npm-windows-upgrade
npm-windows-upgrade

Want to just install the latest version? Sure:

npm-windows-upgrade --npm-version latest