Really? Hitting the red X for closing the update notification window gets a completely new interpretation with the new update roll out. Turning the world upside down, Microsoft decided to interpret hitting red X as “OK, upgrade approved”. Surprise, surprise, the next morning comes the awakening. Even worse the new upgrade takes you to version 1511 which is not reversible.
So if you do not want Windows 10 or need to stay on your current OS this is the time to take action before that window pops up.
And by the way, Windows 10 initial install just started 10 month ago. So why the rush?