I agree with varnie. Updating newer firmware might brick your device.
But still, there is a way how to downgrade.
You can do it over RUNME script or ssh somehow described here
In this link you can download whole Touch system, but I don't know, which version it is.
The rewriting is made by dd if=/mnt/us/mmcblk0p1.img of=/dev/mmcblk0p1 bs=4K
command and it probably delete all personal data. Also it might be dangerous, if you write this command wrong.
This should be fine now.
For RUNME version do this:
1) install data.tar.gz RUNME hack
see this thread to download proper data.tar.gz
2) copy data.tar.gz to root folder of userstore
3) restart Kindle over Menu -> Settintgs -> Menu -> Restart
4) copy again to root folder "RUNME.sh" file with this content:
5) copy to root folder downloaded mmcblk0p1.img
6) restart again