If you have used your Plethora X5 with a Mac or Hackintosh computer then it is more than likely that the Spotlight indexing is preventing you from updating the Plethora. 
Spotlight is a system-wide desktop search feature of Apple's macOS and iOS operating systems. It is designed to allow the user to quickly locate a wide variety of items on the computer, including documents, pictures, music, applications, and System Preferences. 
The easiest way to tell if this is the cause is if there is a folder on Plethora (when connected via USB to a computer) called ".Spotlight-V100" or something similar. 

To resolve this, connect this to your Plethora to your computer and delete all files from the drive and then empty the Trash. 
Once all files are deleted and the trash is emptied, drag the most recent .BIN file onto the drive, disconnect and power cycle the pedal and it should then boot as the most recent version. 

You can download the firmware files from the product page;