watchOS is the only operating system where Apple doesn’t offer a public beta.
watchOS 5 includes several new features such as Walkie-Talkie for two-way person-to-person voice chats, podcast syncing and streaming support, new workout types, pace tracking for runners, an improved Siri watch face, and more.
Apple also released iOS 12 Developer Beta 10 on Friday.
You need to be a member of Apple’s Developer program to install watchOS betas on your Apple Watch. The official public release is coming this fall as a free upgrade for Apple Watch Series 1 and newer.