Alongside the latest beta of iOS 9, Apple has released watchOS Beta 5 to developers. Remember, you'll need to install the watchOS configuration profile from the watchOS Developer Resource Center, and be running the iOS 9 beta in order to test watchOS 2 on a device.
watchOS 2 is expected to see a public release later this year, and it will bring with it a number of improvements for the Apple Watch. Developers will be able to write apps that run natively on the watch, and take advantage of hardware features like the digital crown and heart rate monitor. Developers will also be able to build their own complications for watch faces that support that feature.