How do I replace the main board

as far as i can tell its not an accessible sparepart. however a micro soldering technician might not need one.
if i were you i would find a shop and ask if they would be willing to try.

another (riskier) option would be to find a broken pair with a working mainboard.
sometimes people will sell a pair with a broken headband for 20$

@robertg_69 something like this video will get you there. While you perform this repair/replacement, take lots of pictures and create a guide for the rest of us. It is easy and will really help other people out. Create a new Guide - iFixit

Now, if you need some help with removing the board once everything is opened etc., post some good images of where you are "stuck" so we can assist you further. Adding images to an existing question - iFixit Repair Guide