If you're interested in trying out the latest features, improvements and bugfixes in BusyCal or BusyContacts on macOS, simply follow these steps:
Launch the app and open Preferences.
Click on the Updates tab.
Click on the "including beta" checkbox.
Now click on the BusyCal or BusyContacts menu option (in the top menu bar) and select "Check for Updates..."
When a new stable update is released, you will automatically be informed via the in-app update mechanism.