Remove the ear pads by sliding their edges out of the groove in the frame of the headphones, as shown.

Be careful not to tear the ear pads.

Than you need to use 1 philips head screwdriwer to take out the 4 screws.

You need prye these plastic pint. You need to use a lot of force. The best bethod that worked for me to use a Spudger and a Metal Spudger. But you can try with an iFixit Opening Picks.

After you done, you can see the battery.

The other side is similar. You have to do the same things.