If you want to try it out, you need to sign up through the Samsung Members app. There should be a banner inside the app like the one below. Note that the beta is currently available only to users in the US and South Korea.
Once you sign up, you should see an update for your Galaxy Watch (though it might not appear immediately). It’s a 1.5GB firmware, so make sure that the watch has at least 30% battery since copying it over and installing it will take awhile.
Here’s a changelog for the new One UI 5 Watch software. It brings improvements in three main areas: sleep tracking, activity tracking and user safety.