PWM (pulse width modulation) is the technic widely used for such application.

You may know about it but in case if you do not know about this, you may search this site or on the net. However, just to give you starting point, if you program an MCU output pin to enable external driver in order to supply the power to motor then you can control the speed by means of PWM. If you keep the MCU output pin contineously hi tht will keep motor running at full speed. If you want to reduce the speed you can program that output pin toggle at every 1ms. To be hi for certain time for example 1ms and turn it low for next 1ms,keeps repeating this sequence will ideally provide half the power to the motor that will reduce the speed by 50% theoritically. If you keep MCU output pin hi for 1ms and keep it off for 4ms tht would reduce the speed upto 80% and can give only 20% speed, again theoritically. Exact speed measurement can be done once drivers and motor specs are available. Or just do trial and error.

Rauf