There weren't "dozens of witnesses". There were 9, of whom 7 recanted, one of whom confessed to the crime himself (not something you're likely to do to protect "your homie"). Take your racism elsewhere.
The fact of the matter is that jury convictions, even incredibly unjust jury convictions, are extraordinarily difficult to overturn, to the point of absurdity. The standard of overturning a jury conviction is literal "proof of innocence", which is just plain impossible for the vast majority of innocent convicts. The Supreme Court made their decision 9-0 because the standard of "proof of innocence" is impossible to meet. There may have been twenty years of appeals, but none of those appeals really stood much chance of exonerating the innocent. A big part of the problem is the irrational respect for jury verdicts.
(In answer the original question, declare a mistrial and start all over--he might well have been guilty, but it certainly was never proven beyond a reasonable doubt. Also, no death penalty regardless, obviously.)
There were 34 witnesses, I suggest you start Googling for relevant legal documents and not Amnesty International & NAACP spin spread through media. You could start with the opinion of Clinton-appointed federal judge William Theodore Moore who found miraculous recantations to be "not credible or partly credible" and the Davis's magic unicorn defense as "largely smoke and mirrors".
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