Officially, its not user serviceable, at least this is what Apple will tell you.

But as the teardown shows, once you get below the only point where they used adhesive, you can quite easily replace them. Or could - since you would have to find a replacement part first. But the batteries are not glued down and they do have a real connector! These AirPods Max did get 6 out of 10 Points for being somewhat repair friendly - contrary the the other members of the AirPods family, who earned a whopping… 0 points !