watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-03-12T20:07:33Z. The update package is 64.43 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: 2ef69b03b1f447f47879ab8a20600749e28aa3b1.zip.