iOS 6.1.6 (build 10B500) is an official Apple over-the-air update for the iPhone 3GS, released on 2014-02-21T18:09:29Z. The update package is 757.37 MB. This is a delta (partial) update that requires null already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: 903b149c839c0650fd072d47c52599a52cbdd292.zip.