Downgrading the firmware can be done by sideloading the older version in the same way as manually upgrading it. Going from 2.5.2 to 2.5.1 would be safe.
If you want to go back to much older firmware, then do the factory reset and use the PC to do the setup. It will copy firmware files to the device. Remove these and replace them with the version you want.
https://www.mobileread.com/forums/sho...d.php?t=185660 has the links to the firmware as well as the steps for installing them.