Assuming you are asking about a four-wheel drive vehicle with a manual transmission:
To manually disengage four-wheel drive, first put the car in neutral. Next, turn the knob or lever to the 2WD position. You may need to consult your car's owners manual to find the exact location of the knob or lever. Once in 2WD position, start the car and slowly back up until you hear a click. This click indicates that four-wheel drive has been successfully disengaged.