watchOS 5.1 (build 16R5584a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-10-15T17:09:48Z. The update package is 617.6 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: cfd1c5f8f216ab0d5796f855dd6c92fa2a9b01e8.zip.