iOS 9.3.6 (build 13G37) is an official Apple over-the-air update for the iPhone 4s, released on 2019-07-22T17:06:51Z. The update package is 928.05 MB. This is a delta (partial) update that requires 8.1.1 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: 31e7e91ab0f7729ba1fd82bed11be1836c81c26c.zip.