iOS 6.1.6 (build 10B500) is an official Apple over-the-air update for the iPhone 3GS, released on 2014-02-21T18:09:25Z. The update package is 16 MB. This is a delta (partial) update that requires 6.1 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: b0eb6de8b7d7a6bedadf8e43f5c49077a5494071.zip.