Sorry, there is currently no 3.9 switching pack. If you got an older model (hardware revision A through I. See end of serial number) then you have to buy an RS232/TTL adapter on Ebay or elsewhere to flash your calc in order to downgrade from OS 3.9, then you can install OS 3.1 and/or 3.6 using the existing tutorials. If you got a newer model (hardware J and higher) then you are screwed (at least until Ndless developers figure out a way to install Ndless on OS 3.9, which might take several months).