watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm), released on 2018-09-18T17:23:25Z. The update package is 144.86 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: f52376be6359bb3c640bc38b2078dc4ad189fd61.zip.