watchOS 4.3.1 (build 15T5565a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-05-07T17:23:30Z. The update package is 361.78 MB. This is a delta (partial) update that requires 4.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 8874b5694b3332116a2a13983d26f295a2fa2103.zip.