I don't know that system specifically, but some systems are programmed in the controller itself as a hard speed limit that can't be changed.
Some have a wire that can be cut or soldered or plugged or unplugged to remove the limit, and some don't.
If the vendor doesn't have any specific info on how to change this, it may not be possible, or it might just require experimentation (possibly opening up the controller to see what the pads are labelled, and if there is any info on ES about that unit or one similar enough to make an educated guess.
You could look at the wires, and see if any of them don't connect up to external devices (battery, throttle, motor phase/halls, ebrake, LCD, etc). Then list those (if any) and show them in a pic and maybe we can make a guess what they might be.
Might need voltage measurements or opening up controller to check what pads they go to, etc., before you could try just hooking htem up somewhere, though.
If you're not comfortable with the opening-up or measurements/etc., then once I'm done helping Cvin with that bike, I can help you with yours.