[GeeTwo]

What specifically do you mean?

1. Do you want to reduce the maximum speed that a drive motor can be driven, and scale the lower speeds down proportionally? In this case, a simple multiplier of the motor input values by a number between 0 and 1 will do.
2. Do you want to reduce the motor speed at the low end but allow full speed at the high end? In this case, multiply the motor input value by its absolute value.
3. Do you want to have two or more modes (slow and fast) that you can switch back and forth? In this case, you can use the first case, but make the multiplier dependent on another control.
4. Something else??