watchOS 4.3 (build 15T5199f) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-03-06T18:08:10Z. The update package is 361.92 MB. This is a delta (partial) update that requires 4.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 40da68e06285030cec598cf383366282cba21848.zip.