iOS 6.1.3 (build 10B329) is an official Apple over-the-air update for the iPhone 3GS. The update package is 757.35 MB. This is a delta (partial) update that requires null already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 0525e5023736ea38fd648c440b3d3f8093b6006b.zip.