Question: Is it possible to rename the boards in Plethora X5?

Yes, you can rename them in the board organizer that is part of the TonePrint app. Simply-swipe the board you want to change to the left to reveal the "RENAME" function of your selected board.
You can (if you wish) do this to all boards..  you can also delete all factory content (boards, toneprints) and build your own library of custom toneprints /boards with any name you like.

To download the TonePrint app, simply go to the download center and search "toneprint" to find the latest version here