Chrysler Repair: Repair to the computer, jeep grand cherokee, heavy equipment mechanic

Question
We found out today that our computer is overheating and were quoted $900 to fix it.  My boyfriend used to be a vehicle mechanic and is now a heavy equipment mechanic, so you can talk mechanic jargon and he'll understand.  We had 'all data', but new fees are keeping us from signing on again.  Question is, he is buying a refurbished computer for our 1999 Jeep Grand Cherokee V8.  What are the exact steps to pulling the old computer off and putting the new one on.  Thanks!

Answer
first of all why do u think the computer over heated ?? In all of my years 12 of them I have not seen one over heat  I have seen them catch on fire due to some one doing stuff they were not suppose to.
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Give me some codes or diog that U think it needs to be replaced and what is the issue U are having with it not starting.