Lukasz, here's the schematic:
You can use Arduino Uno or whatever to test, but then to embed something smaller like Nano or Pro Mini is more convenient.
If you look at the source,
https://github.com/alga/shiftlight/b...shiftlight.ino , you'll see that the pin numbers of the input and outputs, as well as RPMs for each led and for the blinking warning are spelled out in the constant definitions.
In order to set the switchover RPMs on a button press you'll need to add a button that grounds one of the free input pins, set the mode for that pin to INPUT_PULLUP, and in the loop check if the pin is LOW, and if it is, fill in the thresholds array with the new values based on current RPM. You'll need to delete the const specifier from the variable definition in order to be able to modify the array. Piece of cake.