Following the beta three releases of iOS 10.2.1 and macOS 10.12.3, Apple today has begun seeding developers with the second build of watchOS 3.1.3 for testing on the Apple Watch. This comes almost a month after beta 1 surfaced, while this new build likely just includes bug fixes out of preparation for a public release sometime in the near future.
To install the latest build of watchOS 3.1.3 on your registered Apple Watch, navigate to the Watch app on your paired iPhone and tap the “My Watch” tab. From there, go to General > Software Update and you should see the update ready for download. Be sure to place your iPhone on Wi-Fi while downloading this software as it’ll surely gobble up plenty of your data.