Good day @Hallo,
Did you buy the 9 Channel controller upgrade? If you didn't you should be able to modify that using the throttle regulators, pointed at in the following image:
Lowering them should help, fiddle with them for a bit until you find what's comfortable. The ones above those are used to adjust the steering.
If you have a 9 Channel controller the process is a bit more hectic, as you'll either have to fiddle with it's innards or (easier) modify the rover's parameters directly, to slow it "manually". If that's your case, just say so and we can get started on it.