The battery is housed in the left half of the headphones (the side with the micro-USB port).

Grasp the left ear cup cushion with your fingers and gently pull towards the center of the earcup. This will release the cushion from the tabs that are holding it in place.

Gently pull the ear cup from the rest of the tabs that hold it in place.

Remove the ear cup cushion.

To reinstall the cushion, align the ear up to the headphones. Squeeze around the perimeter of the cushions to snap the ear cup in place.