iOS 12.3 beta 5 is now available to registered developers, who can download it by going into Settings > General > Software Update on their iPhone or iPad. The update is also available from Apple's Developer Center on the web. Public testers in the Apple Beta Software Program should get the update later this week.
The update to iOS 12.3 is expected to launch to the public later this month with features like an updated version of Apple's TV app. The refreshed app includes machine learning-based recommendations, a streamlined user interface, and a new Channels section that'll let you subscribe to and watch content from services like Showtime, Starz, HBO, Nickelodeon, and CBS All Access.
Apple also released watchOS 5.2.1 beta 5 for the Apple Watch today. There haven't been any major changes discovered in watchOS 5.2.1 yet, so this update may be focused more on under-the-hood fixes and performance improvements.