it can be done either way

this truck does have pre-programmed mode settings but even if they are still faster than you would like then going into the system and manually changing the output speed can be done through the dash.
There is a specific way to do it and a specific SUO parameter that needs to be changed. Without direct knowledge of navigating around in this system and having the setup instructions in hand i would not recommend attempting this adjustment on your own.

And of course with a laptop this can be done in a snap (quite faster than going through the dash)

I know you are trying to avoid calling a cat service tech but i'd recommend it.