A few hours after the final version of watchOS 5 (cf. The final version of watchOS 5 is available for download), Apple is tackling the first beta of watchOS 5.1 which is now available for download.
This pre-release, intended for developers, carries the build 16R5559d. The novelties it brings are not immediately known but should indeed exist; you will just have to wait for the first returns to learn more.
To install watchOS 5.1 beta 1 on your Apple Watch, go to the dedicated application on your iPhone, in the updates section and follow the steps that appear on the screen, if you have the necessary profile for recover beta.