If "nvm" is not installed. In that case, go to https://github.com/coreybutler/nvm-windows/releases
Install and unzip the nvm-setup.zip file.
From cmd type nvm -v to ensure nvm is installed.
Press window key + x --> Windows PowerShell(Admin).  !!V.V.Important
  nvm install 
  nvm list 
  nvm use 
  npm install -g npm
  npm -v

!! Congratulations, you have done it.