Separate the two halves of the headphone.

Be careful not to rip the wires from the speaker off of the circuit board.

Pull the outer part of the hinge away from the inner part on the side without the wire going through it. This will require some force.

Pull the inner part of the headphone partially from the other hinge, being careful not to damage the wire going through it.

If you were to want to replace the battery, now is a good chance to do it.