iOS 6.1.6 (build 10B500) is an official Apple over-the-air update for the iPhone 3GS, released on 2014-02-21T18:09:28Z. The update package is 12.33 MB. This is a delta (partial) update that requires 6.1.3 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: f5fd46b94272da99b23f60ce97ac8531f74e7797.zip.