In August 2015, Samsung launched Galaxy S5 Neo as an affordable variant to the 2014 Galaxy S5 flagship. The Galaxy S5 Neo made its debut running Android Lollipop 5.1.1, with optimized TouchWiz interface. The device was later upgraded to Android Marshmallow and is currently pending Android Nougat update. While the handset is soon expected to get Android Nougat, it is currently getting an update with security patch for the month of May. Earlier the device was running with March security patch.
The recent update is quite smaller in size and comes with build number G903FXXS1BQE1. The update includes vulnerability fixes along with some stability and performance improvements. The update is currently being rolled-out for unlocked devices only via OTA and you should receive it anytime soon.
If you did not receive any notification for the update, you can check it manually by going to Settings -> About Phone -> System updates. You need to make sure that you are on a stable Wi-Fi connection and have sufficient amount of storage space before you start installing the update.