Use a soldering iron to heat up the solder at the points where the red, yellow, and black battery cables connect until they detach from the board.

Detach the connector on the side of the headphone cup to disconnect the red and black cables (next to the battery cables) from the headphones.

Slide the motherboard out of the headphone cup and flip it over.

Careful that you don't rip any of the remaining soldered connections (like the speaker connections for example.)