Though iOS 13 has been available for a week now, Apple today released an iOS 12.4.2 update designed for those who are still running versions of iOS 12 on devices that aren't able to be updated to iOS 13.
Apple is making iOS 12.4.2 available for the original iPad Air, the iPad mini 2, the iPadmini 3, the iPhone 5s, iPhone 6, iPhone 6 Plus, and sixth-generation iPod touch, all of which are not compatible with iOS 13.
The iOS 12.4.2 update can be downloaded on eligible devices over the air by opening up the Settings app, selecting the General section, and tapping on Software Update.
It's not clear what's included in iOS 12.4.2, but it is likely a security update that adds important security fixes for older devices that weren't able to get those updates through the newer software. Apple's release notes say the update "includes improvements, provides important security updates, and is recommended for all users."
A security support document provided alongside the update indicates that it fixes a vulnerability that could allow a remote attacker to case unexpected application termination or arbitrary code execution.
Source: MacRumors