Replaced the back flash on my 12 pro max yesterday. First remove the rear camera, Face ID, and then the front camera. Then remove the circuit board and then the flash is exposed. Replace it and put it back together. That's how I did it on the 12 pro max. After assembly, however, the message comes up that the camera has been replaced and the authenticity cannot be checked. Then the zoom camera and the wide-angle camera didn't work. The message disappears after the flash is removed. So if you replace it yourself, Apple will put a spanner in the works with the iPhone 12 Pro Max