Step 1 : Delete "npm" folder from the following path
C:\Users\YourUserName\AppData\Roaming
Step 2 : Once you have the "npm" folder deleted, uninstall Node.Js.
Step 3 : Reinstall Node.JS
Step 4 : Install Angular CLI Using this command npm install -g @angular/cli@latest
Step 5: Now try : ng --version
or ng -v