iOS 6.1.3 (build 10B329) is an official Apple over-the-air update for the iPhone 4s. The update package is 87.37 MB. This is a delta (partial) update that requires 6.0 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: fed59972bbaf5105b0fde520f36428f6078d5961.zip.