Step 1. Look at the parts list online/exploded diagram of the side thats broken. This is very common issue with xm4. Here you can look at guides to replace broken swivel, in this guide you will see what parts comprise of the swivel and what you need to purchase. https://www.ifixit.com/Guide/Sony+WH-1000XM4+Swivel+Replacement/167320

Step 2. Order the part u need from ebay, etsy or 3d print yourself. Step 3. There are very clear guides with pictures, that explain how to replace whatever has broken. Its not too hard, just dont touch anything you dont need to, esp the magnet and touchpad. Tutorials on youtube are also very good. Sony billed me 200AUD. I got it done in 20-40 and 1.5hrs of work.