watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-03-12T20:07:23Z. The update package is 324.4 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: ca77bee18cdc6012171f48692e356778836f01bd.zip.