FirmWare 3.31 is for the PowerFlarm core.
It has nothing to do with the Butterfly display.
So, pushing buttons of the display at power-up is irrelevant for the CORE firmware update.
Holding the display push button down at powerup is only relevant for DISPLAY firmware update or new FlarmNet database upload.

Take a clean USB stick
(as mentioned above, nothing fancy like USB3, a Gazillion GB, properly formatted in FAT32)
Make sure there are no other firmware files (.fw) in the root (top) directory.
Watch the LED on the Core blinking.
When the blinking stops and the standard screen (butterfly with green square GPS and TX boxes) comes up, the process is done.