watchOS 5.1 (build 16R5584a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-10-15T17:09:46Z. The update package is 782.34 MB. This is a delta (partial) update that requires 4.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 7a585275862632a6983cdab65f9ca82bc6fddd2d.zip.