Samsung starts rolling out Android 7.1 Nougat to its Galaxy On 5 (2015) and On 5 Pro (2016). Its good to see that Samsung is taking care of even 2 years old phones. The update comes with the firmware build number G550FYXXU1CQL6. The firmware build is same for both the phones.
For a reminder, Samsung Galaxy On 5 comes with Android Lollipop out of the box and Galaxy On 5 Pro runs on Android Marshmallow out of the box. The update weighs around 1 GB in size. Which is packed with Android 7.1 Nougat with all its goodies like split screen multitasking, enhanced doze implementation and more control over notifications. Along with Android Nougat this update also contains some UX improvements and adds various performance modes.
Coming back to the update, as it is an OTA update it might take some time to reach each and every users. However, you can also check the update manually by going through the settings>> about phone>>software updates if the update notification is not popped up on your phone.
Note: As the update is large one, so use Wifi network to download the update to avoid data charges. Moreover, charge your phone up to 50 % before installing the update.