Ubuntu
Open Terminal. Type:
sudo sed -i "s/NoDisplay=true/NoDisplay=false/g" /etc/xdg/autostart/*.desktop
gnome-session-properties
When the "Startup Applications Preferences" window pops up, under "Additional startup programs:", scroll down to:
Update Notifier
Check for available updates automatically
On the left, disable the checkbox for "Update Notifier". Click "Close".
Source:
Lubuntu
Open LXTerminal. Type:
sudo sed -i "s/NoDisplay=true/NoDisplay=false/g" /etc/xdg/autostart/*.desktop
lxsession-edit
When the "Desktop Session Settings" window pops up, in the "Automatically Started Applications" tab, under "Applications automatically started after entering desktop:", scroll down to:
Update Notifier | Check for available updates automatically
On the left, disable the checkbox for "Update Notifier". Click "OK".