I use the Ubuntu version of Pi Imager, here is the windows version
https://downloads.raspberrypi.org/imager/imager_latest.exe
Under custom you can select the khadas image from your download location and install.
You will have to use the button press sequence in the manual to force it to boot from SD if it still has android installed. Pretty sure that after you overwrite emmc it will autoboot from SD.
Also, if you are using a VM like virtualbox in windows to host ubuntu you will have to map your usb drive to that instance of VM or the port will not work.