watchOS 4.3 (build 15T5212a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-03-16T20:09:19Z. The update package is 324.42 MB. This is a delta (partial) update that requires 4.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: b89de59d73d64c2283d3f571e14d56985604fe10.zip.