watchOS 4.3.1 (build 15T5545b) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-04-16T17:04:22Z. The update package is 326.72 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: 4f24e3ad7ffba9367a1b68cd23b8d11b08513c5f.zip.