You question does not say what size DC motor you are wanting to control, so this answer will have to be generic.
First of all, you DON'T drop the voltage "OF" the DC motor, you drop the voltage "TO" the motor.
The simple answer is to place a "RESISTANCE," or "RESISTOR" in series with the motor. A resistor limits the amount of current flowing through it, and thus the voltage.
If you are talking about the very small DC hobby type motors [designed for 1 to 3 volt inputs], one way would be to use a variable resistor, like a large potentiometer ["pot"], scavanged out of a discarded "old" TV or "old" radio [both "pre" solid state era].
Another trick I've used [requires a lot of trial and error] is to use an assortment of old [but working] incandescent/filiment type light bulbs [small, like from flashlights, Christmas tree bulbs, dial lights from "old" radios and TVs, etc.] as resistors.
You can experiment around with "stacking" these in series to add more resistance, with each addition slowing the motor more. And if even the largest of them slows the motor TOO MUCH, then they can be wired in PARALLEL to decrease resistance, allowing more current flow. As said above, this can require much trial and error, but it can be made to work.
Another method [if using batteries] would be to use fewer batteries in series. This only works for larger voltage motors, like out of 24/18/12/9/7.2 volt motors [like out of cordless drills for example].
As an example, consider using an 18 volt motor. To provide the "normal" 18 volts would require 12 1.5 volt alkaline batteries in series. If you used only 6 batteries, you would have reduced the voltage by half. By varying the number of batteries, you would be able to vary the motor speed. Remember however, that there is a limit to how much you can "starve" the motor and still have it run or do any work.
If your aplication allows you to use an AC power source, then another method [much more expensive] would be to use a benchtop AC powered, variable power supply.
Another common method is pulse width modulation (PWM). The full available voltage is switched on and short time later it is turned off again. This cycle repeats continuously at a relatively high frequency (hundreds, to tens of thousands of times per second). The ratio of the on-time to the off-time determines the apparent percentage of the DC supply that the motor sees.
The circuits to drive motors this way can be relatively simple to extremely complex, depending on the size of the motor, available DC voltage, and other controls needed (such as controlling speed instead of voltage supplied to the motor).
ANSWER: motor are not voltage related but rather power related so the answer is not to reduce the voltage but to reduce the power going to it. REDUCE THE POWER WILL REDUCE THE SPEED. either reducing current and /or voltage will do the trick.