composer require magento/product-community-edition 2.2.0 --no-update composer update
But while running this command I get the following error:
Updating modules: Unable to unserialize value. [InvalidArgumentException] Unable to unserialize value.
I read that multiple developers had this problem after upgrading to Magento 2.2 and it could be a problem with installed extension, but for me there isn't any entry in the log files of Magento, PHP or MySQL.