watchOS 4.3.1 (build 15T5567a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-05-14T17:14:11Z. The update package is 361.82 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: 5d5c29b0ce373719e528a4ab112637754a557cf4.zip.