Extract the root filesystem image and kernel image from amazon firmware update version of your choice (using "kindletool"). Then boot to diags (with the "rescue pack" or "cowards rescue pack" method), then flash the main kernel and rootfs as described in the "Simple Debricking" thread, starting at the "diags" step, here:
https://www.mobileread.com/forums/sho...d.php?t=170929
You can use a Kubrick boot CD (or bootable USB stick) to get a root shell where you can do this, if you have a windows PC. Or use whatever SSH tool your host OS supports. On windows, I use Putty for SSH.