The Koryū is powered by an Atmel Atmega328P microcontroller (MC).
You basically just need some way to re-flash that. The USBISP compatible ones mentioned are easy to come by and quite cheap (eBay is your friend). Which software you use to reprogram the MC does not matter, and several different are available. I myself use a Bus pirate as a programmer and the open source ‘avrdude’ software, but the end result is the same.
If you just google something like “Atmel microcontroller programming guide” you should find a lot of easy to follow guides to help you.
As mentioned in the other reply above, you also need to make sure that you get the six pin female ISP cable to connect your programmer to the MC. Make sure to verify the pin out not to break anything.
Atmel MCs are rugged, so it should be quite hard to brick anything beyond recoverability, but double check all connections just to be sure.