You just have to download the previous version of FlashPrint. I think it's v3.2.0 which you can find here (http://www.flashforge-usa.com/support/downloads/
). After you download and install it, don't open the program just yet. First, browse to the installation directory and you should see a "Firmware" folder that contains the files you need. Copy that to a new location on your PC, since sometimes the auto-updater in FlashPrint will overwrite the old firmware files with the new ones (which you don't want!)
So then go ahead and open FlashPrint. It may ask you about an automatic update or something like that. It's fine to accept if you want since you already saved the firmware files you need.
Finally, close FlashPrint, copy the old firmware files back into the installation directory, and then open FlashPrint again. You can manually flash the firmware using the files you just copied over by going to Tools > Update Firmware.