watchOS 5.1 (build 16R5577c) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-10-09T17:12:07Z. The update package is 783.36 MB. This is a delta (partial) update that requires 4.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: a89d1956d7536374768b40fad6aa7c02b2696713.zip.