watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-03-12T20:07:26Z. The update package is 143.41 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: 6f92a40684cbe266c328ee62158756bf637c3028.zip.