Apple security update seems to only be for newest members of the iPhone family.
Apple released a software update for users of the iPhone 7 and iPhone 7 Plus only. The description of the update reads:
iOS 10.0.3 fixes bugs including an issue where some users could temporarily lose cellular connectivity.
For more information on the security content of this update, visit support.apple.com/en-us/HT201222.
The update is only 75.4 MB in size and it only took me about 1 minute to complete the download and about 5 minutes to finish the update process. At this time, it appears that this update was only released for iPhone 7 and 7 Plus users.