Is MageRewards installable via Magento Connect?
No, as of MageRewards 22.214.171.124, released on 10.07.2013, updating MageRewards through Magento Connect is no longer possible. You will need to update via FTP.
For instructions on how to do this, please refer to section 2b of the following guide MageRewards Installation Guide for Magento.
If MageRewards was initially installed via FTP, you cannot use Magento Connect to update to the latest version.