watchOS 4.3.1 (build 15T5535b) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-04-02T17:13:17Z. The update package is 326.66 MB. This is a delta (partial) update that requires 4.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: a9ac42f38d637dac462901e18db130c9f557653b.zip.