watchOS 5.1 (build 16R5577c) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-10-09T17:11:51Z. The update package is 732.15 MB. This is a delta (partial) update that requires 4.3.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 8f4e38df34e40953f87fb47c8014e1c7a60eb210.zip.