watchOS 5.1 (build 16R5584a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm), released on 2018-10-15T17:10:25Z. The update package is 147.48 MB. This is a delta (partial) update that requires 5.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 9b0d360f1d59f2f4794ed924e25d30b8b3dd672f.zip.