note that i just tried this and am not sure if it actually works:
- sudo nano /etc/xdg/autostart/update-notifier.desktop
- change "NoDisplay=true" to "NoDisplay=false"
- open "startup applications"
+ uncheck 'update notifier'
- open dconf
+ go to com > ubuntu > update-notifier
* change regular-auto-launch-interval to a really high number