Apple has released the second beta of the next update to WatchOS, WatchOS 5.1.3. The update is available to developer testers only as it requires a configuration profile from the Apple Dev Center. The build comes a week after the first beta was released.
Participating in the beta program for WatchOS is a bit more tricky than it is for iOS or MacOS as it requires a configuration profile on the watch. It is not for the faint of heart and why Apple has limited it to developers only.
WatchOS 5.1.3 is a maintenance release and likely will not contain many if any new features. The focus appears to be on bug fixes and overall stability in the platform. That was the main focus of the 5.1.2 release a few weeks ago although it did bring the much anticipated ECG feature to the Apple Watch Series 4.
It is likely that we will not see this new build of WatchOS go out to the general public before mid-January 2019. Apple usually does four to six beta builds of a release before it goes live for everyone. That, of course, can shift if there is a show-stopping bug found in the beta testing process.
If you are curious or interested in becoming an Apple Developer and testing out this new build, you can get details at the Apple Developer Center.