It is annoying sometimes. Use this simple freeware named Never10 to disable it.
This program will only change some configuration of windows to disallow windows to display the notification.
Excerpt from the website:
The elegance of this “Never 10” utility, is that it does not
install ANY software of its own. It simply and quickly
performs the required system editing for its user.
Windows 10 free upgrade will end on 29 July 2016