Presse RESET and SETUP buttons of your board, release the RESET and keep SETUP pressed until the LED start blinking on yellow color, then release it as well.

Now you can flash it using DFU command:

$ sudo dfu-util -d 2b04:d006 -a 0 -s 0x08020000 -D nuttx.bin

Download “nuttx_base64.txt”: https://gist.github.com/acassis/4479c13e029415cd05a4b52493f1da52

$ cat nuttx_base64.txt | base64 --decode > nuttx.bin

Confirm the file was created correctly:

$ md5sum nuttx.bin
56f9b321ca946f092e89f42723a0cea3 nuttx.bin