Apple recently released their iOS 14.5.1 software update which fixed some security issues, now they have also released iOS 12.5.3 for older iPhones.
The iOS 12.5.3 software update has been released for older iPhones that are not supported by the new iOS 14.5.1 software.
This update fixes some security issues in WebKit that were also found in the most recent release of Apple’s iOS software.
The iOS 12.5.3 software update is available for the iPhone 5s, iPhone 6, iPhone 6 Plus, iPad Air, iPad mini 2, iPad mini 3, and iPod touch 6th generationm and you can download the update from the settings menu on your device.
Source Apple