watchOS 5.1 (build 16R6591) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-10-30T17:21:13Z. The update package is 578.82 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: 8f28b8eeddb5e824460e2eeb7b256a96037ff5b1.zip.