watchOS 4.3.1 (build 15T5567a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-05-14T17:14:15Z. The update package is 361.92 MB. This is a delta (partial) update that requires 4.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 135baecef483393ad973c1c9422a0c6b017f1368.zip.