How do i program a computer and replace new one on 1996 Cadillac Seville SLS

computer keeps sendind all kinds of messages

Replacing a computer should only have be done in few cases. It should has to be done by a trained technician and only after all proper diagnostics have been done first. Without a proper diagnosis many parts are replaced unnessasrly.
That is entirely correct, just because you have this done to an ECM, doesn't make all your problems go away, they should be fixed then and only then should this be done,as a last resort. If your problems are not solved first, they will still show up with the new one as well.
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It is best to take it to a dealer and ask them to "flash " it for you, what this means is,they will clear the old one out and put the correct information on the new one. The dealer is the only place that can do this besides the factory to my knowledge.