The first step is to know how we can disassemble the headphones to reach the battery and the 3 buttons on the left. Hovewer, there are torx screws on each side, but they can't reached because of the speakers. So let's take a look at the picture.What if we first disassemble the speakers, and then we maybe could take off the speaker's covers, and finally reach the torx screws what we need?
I just note, the headphone in the picture is placed "inversely", so the left side is in the right, and the right side is on the left.