Firmly grip the ear pad and the plastic back plate underneath. With your other hand, grip the back of the speaker casing.
Do not grip the back of the speaker by the headband. Twisting this connection will break the headphone off the headband.
With the ear pad facing you, push down on the back plate and turn counter-clockwise to unlock the prongs holding it in place. The back plate will only rotate about 1 cm.
Lift the back plate (with the ear pad attached) straight up from the speaker.
This should be very easy to do. If it doesn't come off easily, then the plate isn't fully rotated.
Remove three 7 mm Phillips #00 screws holding the speaker in place.
Carefully open the back casing, exposing the speaker wires.
Use wire cutters to cut the blue and black wires from the headset close to where they are spliced to the red and black wires from the speaker.
Splice the headset wires to the corresponding replacement speaker wires.
Black connects to black, red connects to blue.
Although soldering isn't necessary when splicing wires, doing so creates a more reliable connection.