The power of a stepper motor is regulated by the input voltage and the current. The input voltage generally cannot be adjusted, as other components in the 3D printer depend on it. But the current can often easily be adjusted. Depending on the type of stepper driver, the current may be adjusted differently.
On Pololu style stepper drivers and 3D printer controller board, you will find a small set screw, called a trimpot. By adjusting it you reduces or increase the current, depending on the stepper driver. Be careful adjusting the trimpot, it may break easily. Always disconnect power before adjusting the trimpot.
Bigger stepper drivers may have dip-switches (small white switches), that change the current. Check the datasheet for more information on how to set the switches.