Firmware upload
1.With your windows PC
Step 1: Connect 24V (Power on the board)

Step 2: Connect USB-C cable to your Klippe device (usually RaspberryPi)

Step 3: Push and hold the BOOTSEL button

Step 4: Push the RST button and hold 0.5 seconds

Step 5: Release the RST button, after 3 seconds, Release the bootsel button

Step 6: RPI-RP2 folder will show up on your computer, copy your built firmware klippe.uf2 to the folder.

2. With your Pi
Thanks for mk-maddin's work:

Connecting the board and bring it into flash mode
Step 1: Connect 24V (Power on the board)

Step 2: Connect USB-C cable to your Klippe device (usually RaspberryPi)

Step 3: Push and hold the BOOTSEL button

Step 4: Push the RST button and hold 0.5 seconds

Step 5: Release the RST button, after 3 seconds, Release the bootsel button

The more detail please reference FYSETC-ERB on Github.