Alongside the launch of iOS 12.4, Apple today released new versions of iOS 9.3.6 and iOS 10.3.4 for older iPhones and iPads that aren't able to run iOS 12.4.
The iOS 9.3.6 update is available for the iPhone 4s and cellular models of the original iPad mini, the iPad 2, and the iPad 3, while iOS 10.3.4 is available for the cellular version of the iPad 4 and the iPhone 5.
These updates can be downloaded on eligible devices over-the-air through the Settings app. To access the software, go to Settings > General > Software Update.
Apple released these older software updates to address an issue that could impact GPS location performance and lead to the system date and time being incorrect. In a support document, Apple says that starting in November 2019, some iPhone and iPad models introduced in 2012 and earlier will require this update to maintain an accurate GPS location due to the GPS time rollover issue that began affecting GPS-enabled products from other manufacturers on April 6, 2019.
Apple recommends all users with eligible devices install the new updates. Apple devices won't be affected until November 3, 2019, so there should be plenty of time for users with affected iPhones and iPads to install the new software.
According to Apple, those who don't update to the new version of iOS by November 3 might see their iPhones and iPads failing to maintain an accurate GPS position, and features that rely on correct date and time, like syncing with iCloud and fetching email, might not work.