First of all, this is a cumulative update that contains all security and non-security patches from all previous updates. In a TechNet blog post Monday, Microsoft warned that failure to install this update "will prevent Windows Update from patching your system with any future updates," starting with those currently scheduled to arrive in May. When that comes out, perhaps we'll call it Update 2. Important: This update will only work for systems already running Windows 8.1. Systems with prior versions will have to be upgraded to 8.1 and then have the Update applied.