A prerequisite for upgrading Magento 2.0 is:
*For upgrade or update, you must use the same authentication keys you used to install the Magento software. For example, you cannot use Magento CE authentication keys to update or upgrade Magento EE or vice versa.*
How or where can I find my Authentication Keys?
Seems they do not exists within the local copy of Magento. You need to login into Magento Connect via web browser to get them from the "cloud".