watchOS 4.3 (build 15T5165e) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-01-25T18:04:23Z. The update package is 613.82 MB. This is a delta (partial) update that requires 3.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 83782ca0a990774b1a252e3857e1e062aff4f1eb.zip.