Download latest BIOS: https://pcengines.github.io/

Download TinyCore http://pcengines.ch/file/apu2-tinycore6.4.img.gz

Make sure the USB drive is not mounted (lsblk or mount command).

Write TinyCore and BIOS image to USB:

gunzip apu2-tinycore6.4.img.gz
sudo dd if=apu2-tinycore6.4.img of=/dev/sdX bs=4k oflag=direct status=progress

sudo mount /dev/sdX1 /mnt
sudo cp apu2_v4.19.0.1.rom /mnt
sudo umount /mnt

Connect USB drive to APU2

Connect APU2 via serial:

screen /dev/ttyUSB0 115200

Connect power

Update BIOS

# cd /media/SYSLINUX
# flashrom --programmer internal -w apu2_v4.19.0.1.rom