Fahrbike Terra - How to reset pedal assist power levels?

First, you look thru the menu on the display. Next, you google for the manual for your bike, and read it.

Last, would be to look for this important capability to be available, BEFORE, rather than AFTER, you purchase the bike.

Since that ship has likely already sailed, you may want to research displays that might be compatible with your controller, and/or with your new controller, AFTER you research what controllers are compatible with your motor and your battery, making sure to examine those displays, and possibly controllers, who have this particular ability you consider valuable and/or necessary.

One possible workaround would be to change the wheel size, since the circuit decides speed by number of wheel revolutions, you can at least varry the assist level from what it is now, if not control it as you would wish.
Recent communications with Fahrbike confirm it is not available with this display.
Surprisingly, Magiccycles are one of a few to offer this.
It would appear further investigation along the lines you suggest is in order.