Gently lift or pry away at the thin parts of the headband arch to separate the thin case segments from the deep case segments.

This part of the arch breaking is an incredibly common point of failure on these headphones, this step may not be necessary (and why I don't have pics of the process to offer myself!)

Gently separate the two, but do not fully separate them as they are still connected by the cable (which is held in with retaining brackets).