iOS 6.1.6 (build 10B500) is an official Apple over-the-air update for the iPhone 3GS, released on 2014-02-21T18:09:14Z. The update package is 16.04 MB. This is a delta (partial) update that requires 6.1.2 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: cbdba3187bc76101b330cebc9fa6b14fe64aa3b2.zip.