How to install Drush 8 on Windows 10?
Finally...

using this command

 composer global require drush/drush:8.*


But i need to refer few steps before running this command.

  composer.json  # Removed the line containing drush like below



  {
          "require": {
              "drush/drush": "7.*"
          }
      }


composer global update
composer global require drush/drush:8.*


source https://github.com/drush-ops/drush/issues/2006