watchOS 4.3.1 (build 15T567) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-05-29T17:09:31Z. The update package is 370.8 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: c28a5420e3451e6562b0799259d3163810f4673e.zip.