iOS 13.3.1 (build 17D5044a) is an official Apple over-the-air update for the iPhone 6s, released on 2020-01-14T18:21:11Z. The update package is 189.59 MB. This is a delta (partial) update that requires 13.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 076786af5e596099df6cd9147eb7ecbfaa98279b.zip.