watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-09-18T17:22:41Z. The update package is 505.53 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: d9a970baf239722676f87f1cb6587847bfe62ee9.zip.