Auto firmware builds are posted here. Please be sure to update straight away, the firmware shipped in the preview boards has a ton of usability bugs that I’ve cleaned up this week.
Press and hold the button on the bottom of the Bus Pirate near the flash chip. Plug in the USB cable. It will connect as a USB drive. Drag and drop the firmware .uf2 file into the drive. After a short update, the Bus Pirate will reset and connect.