How Do I Disable iOS Software Update Notifications?
As of iOS 9.2, there is no good solution to disable the reminder that appears when iOS updates are available and ready to be installed. The updates are automatically downloaded when you are connected to WiFi, so theoretically if you never connected to WiFi from your iOS device then the updates wouldn't download automatically (you can delete an update that's already been downloaded by going to Settings > General > Storage & iCloud Usage > Manage Storage).
Of course avoiding WiFi isn't a practical solution for most users, so the best way to get rid of the notification is to just go ahead and update your device. I always recommend backing up your device first, of course.