The dryer plug adapter cord needed to change an old three-prong plug to a new four-prong plug is available at hardware stores, such as Lowe's or Home Depot. After obtaining the cord, unplug the dryer and move it into a position so that the back is easily accessible.
Remove the electrical terminal cover plate, located where the cord enters the dryer. Visually inspect and note the color of the wires attached to each terminal and their associated colored screws. The red and black, hot, wires have brass screws and the neutral has silver. Remove the wires and the ground strap, which is a plate or wire connecting the center terminal to the dryer frame.
Insert the new wire into the terminal leads making sure that the appropriate wire color is replacing its predecessor and that the same color of screw is used in each. The black wire is usually on the left with the red wire on the right and the white in the middle. Make sure to fasten the wires snugly. The green wire should remain unattached at this point.
Drill a 1/4-inch hole in the dryer frame, within reach of the green ground wire. Using a ground screw, attach the lead from the green wire directly to the dryer frame. Replace the cover plate, plug the unit in and test operation.