watchOS 4.3.1 (build 15T6567) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-05-29T17:09:20Z. The update package is 158.4 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: ff19f9fdb59923a46c329414c544e437ded74ebb.zip.