iOS 6.1.6 (build 10B500) is an official Apple over-the-air update for the iPhone 3GS, released on 2014-02-21T18:09:09Z. The update package is 71.8 MB. This is a delta (partial) update that requires 6.0.1 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: 2dc801dfda2d10c6acdd1b7a88b32a7fd420aeb3.zip.