watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 2 (42mm), released on 2018-09-18T17:22:37Z. The update package is 369.34 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: 85c6056fdc497bf2a4476a36b32b9fa8430f6ded.zip.