watchOS 5.1 (build 16R5589c) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-10-22T17:19:16Z. The update package is 777.7 MB. This is a delta (partial) update that requires 4.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 583b62f13fc2885b29c16ac1baeecbfba563dffa.zip.