I do upgrding the following way:
Unfortunately, that did not work. Previous version was Magento 2 Merchant Beta (master branch) with sample data installed. I had to remove composer.json and composer.json.lock to satisfy git pull (maybe there's a better way?). When running setup:upgrade, this error occurs:
PHP Fatal error: Cannot instantiate interface Magento\Store\Api\StoreRepositoryInterface
Is there an established path to go from Merchant Beta to RC with sample data installed?