watchOS 4.3.1 (build 15T5535b) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-04-02T17:13:26Z. The update package is 92.88 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: 1c50cae08640891939a956446ff65ad77494b025.zip.