iOS 9.3.6 (build 13G37) is an official Apple over-the-air update for the iPhone 4s, released on 2019-07-22T17:06:52Z. The update package is 857.06 MB. This is a delta (partial) update that requires 8.3 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: 7742d113402acd6d3d501aebb21d01954b047d2e.zip.