watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-09-18T17:22:48Z. The update package is 447.14 MB. This is a delta (partial) update that requires 5.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 0bfcab56d26bf0c69067b1a3f8e708a8b1e58216.zip.