I'd go with a factory reset and a reinstall/update of a full official update, that should mostly get it back to factory status.
Depending on the current FW version (>= 5.4.2.1), you might have to look at the 'special' downgrading instructions. (They're in a thread somewhere, I should really keep the link handy, because it keeps popping up ;D).
EDIT: There:
https://www.mobileread.com/forums/sho...=235413&page=2