While users may still be able to download and update their iPhone 5 and 5C devices, the OTA updates for the latest version of iOS, iOS 10.3, has been pulled by Apple for the aforementioned devices.
Apple released iOS 10.3 for iPhone 5 and later on Monday and it includes a whole host of improvements and new features, among which Find My AirPods, Apple File System implementation, and updates for devs and App Store are the most prominent. It was recommended by a number of sources to download and install iOS 10.3 for the devices that support it due to the extra zip that it provides, but iPhone 5 and 5C users will not be able to do so using the Over the Air (OTA) method on their devices.
That is not to say that iOS 10.3 is completely beyond their reach, as the traditional iTunes method still works; the procedure also remains the same: connect your iPhone to Mac or PC using the lightning cable, wait for your device to be detected, and iTunes should automatically prompt you to download and install the latest version of iOS available for you device. Even if it doesn’t because automatic prompts are turned off, you could open iTunes, click on the small iPhone icon, and click on Check for Updates to do the same.