watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-09-18T17:23:04Z. The update package is 412.19 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: af680323e2ee163b7e9b04396ab58d40800c05a5.zip.