If you have to ask, you probably don't have the skills to do it. Desoldering and soldering a new APU on the PS4 is going to require some serious, expensive equipment, and isn't usually the way most people would fix a BLOD anyways.
Uhhh well take it to someone who knows what they're talking about, the APU won't get "corrupted". BLOD is usually caused by one of two things, the solder used for the APU gets oxidized and causes a bad connection, or sometimes you can get BLOD from a bad HDMI port/controller.
To fix the former, you generally just reflow the solder using something like a rework station. You would basically never desolder and resolder a new APU. You could attempt to reflow the solder in an oven, but I don't recommend it.
His words were somewhat strange, that he had done a reballing of the processor, and that whoever used the device previously was not interested in the heat and was playing with the device, and that the processor had been damaged, he has a reballing machine, and I do not know whether he is honest in what he said or is lazy about repairing.