Latest beta can be downloaded from the release announcement.
Use the Amazon Fire OS Devices version.
Uninstall whatever version is installed, sideload the beta, then restart the FireTV device.
I installed the beta on my FireStick 4K Max yesterday. I had to uninstall the app store version first
Install failed when I tried to install the beta on top of the app store version. Not sure if the restart is really required. Just force of habit on my part.
