Just sideload the official Amazon update.bin -- as usual, except that nowadays the framework detects a downgrade attempt, so you must trick it into using the boot-time updater (which does not have this problem) rather than the framework updater.
In order to use the boot-time updater, you must do the following:
- Sideload the update, via USB. DO NOT EJECT OR UNPLUG THE KINDLE!!! This would allow the framework to see the update, which is BAD.
- Flush the disk cache, ensuring the file has been fully written to disk. (Normally ejecting would do this, but we do not want to eject.) On linux, you simply type "sync" on the command line. There are Windows utilities you can install which do the same thing.
Alternatively, wait a few minutes after the file finishes copying. The OS will probably have written the file to disk at that point.
- Hold down the power button on the Kindle, again without ejecting it. Your Kindle will restart. Pull out the USB cord at this time. Since it was in USB mode the entire time since the update was copied over, the framework never saw and deleted the update.
Your Kindle will now perform the update to the downgraded firmware.