Turn off and unplug your cassette deck. Place the cassette deck face-down on a work surface. Pry off the rubber feet on the bottom of your cassette deck with the utility knife. Remove all screws from the bottom panel with a screwdriver.
Turn the cassette deck over placing the bottom side down on the work surface. Remove all the screws on the front and sides of the deck using a screwdriver. Remove the outer casing of your cassette deck.
Look for the motor, which is generally next to the power supply. Locate a small silver screw that controls the potentiometer, usually located next to the motor. Turn the screw slightly to the left with the flat-edged screwdriver.
Replace the outer casing on your cassette deck. Insert all the screws and screw them back in. Plug your cassette deck in. Plug in your headphones into the headphone socket.
Switch on your cassette deck. Insert a cassette and press "Play." Put the headphones on.
Listen to your cassette and determine whether it is playing faster of slower than before. Repeat the previous steps if you wish to further speed up or slow down your cassette deck, adjusting the silver screw either left or right once you've determined whether turning it left slowed or sped up your cassette deck.