watchOS 5.1 (build 16R5577c) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-10-09T17:11:50Z. The update package is 732.27 MB. This is a delta (partial) update that requires 4.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: f50515ecbb0d02ecae66b931d8d779f0d2b8a525.zip.