watchOS 4.3.1 (build 15T5555c) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-05-01T17:06:46Z. The update package is 325.78 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: 52cbff00682828674ebfb13ddf4a3c5500801761.zip.