watchOS 4.3.1 (build 15T5565a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-05-07T17:23:32Z. The update package is 134.42 MB. This is a delta (partial) update that requires 4.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 49f91846570bcf5089d521698ca1c32aaab0323b.zip.