watchOS 4.3 (build 15T5212a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-03-16T20:09:23Z. The update package is 64.41 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: 10be3bf78bc02453b3458ed8f80d8be6d6a2e36d.zip.