watchOS 5.1 (build 16R5568d) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-10-02T17:14:30Z. The update package is 734.55 MB. This is a delta (partial) update that requires 4.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 6a38f01761a72fbc8aec81c8ea48653a7f7bdfbb.zip.