watchOS 4.3 (build 15T5165e) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-01-25T18:04:18Z. The update package is 328.25 MB. This is a delta (partial) update that requires 4.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 4b845c513a3334e79db10b06e19176711890e210.zip.