watchOS 5.1 (build 16R591) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm), released on 2018-10-30T17:21:51Z. The update package is 159.25 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: 47e9f35e36f4df43139782c07d73b36f8862acc6.zip.