Let a specialist do it. A few years ago I gave the ECU to my electronics teacher who himself is a car enthusiast and he had offered to solder the new chip in for me. He gave me the ECU the next day saying he stopped before he got any further because he said it's a very difficult job, and said there are loads of copper tracks running under the chip so he was going to eventually ruin it. I then passed the ECU on to Danny_p who is an electronic wizard and he ended up ruining the ECU as it's just too sensitive to use a soldering iron to chip it.
I'd go to a specialist, who has the right equipment and the right knowledge.