watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2018-09-18T17:23:26Z. The update package is 160.48 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: bde9c92feda0e72998802bd784b7425a9248a6b2.zip.