Samsung released One UI 5 Watch a month ago and the first beta is now available. It works with Galaxy Watch4 and Watch4 Classic as well as Galaxy Watch5 and Watch5 Pro.
If you want to try it out, you need to register through the Samsung Members app. There should be a banner inside the app as shown below. Note that the beta is currently only available to users in the US and South Korea.
Browse this banner in Samsung Members to sign up for One UI 5 Watch beta
After signing up, you should see an update for your Galaxy Watch (although it may not appear right away). This is a 1.5GB firmware, so make sure your watch has at least 23% battery after copying the installation It takes a while.
One UI 5 Watch Highlights
This is the changelog for the new One UI 5 Watch software. It brings improvements in three main areas: sleep tracking, activity tracking, and user security.
First One UI 5 Watch beta changelog