watchOS 5.1 (build 16R5577c) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-10-09T17:12:03Z. The update package is 777.37 MB. This is a delta (partial) update that requires 4.3.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 479ff9bb331544fd21310bedac4ea9ee1ed13173.zip.