watchOS 5.1.2 (build 16S5042a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-11-07T21:14:57Z. The update package is 451.33 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: 895777a3cc635d2cf6ba3cf3ea4c2375db9cfb69.zip.