Re-Training

Question
There is dog up for adoption where I live that was train to attack humans is their a way for me to help him unlearn what his previous owner trained him to do.

Answer
Only is if the behavior is under stimulus control.  If the dog will truly only "attack" when given a precise cue, then if you never give the cue you shouldn't have a problem.

HOWEVER, the reason many of these dogs are trained in this manner is that they already have a predisposition for aggressive responses.  I would want to know the history of the dog, the qualifications of the trainer and any bite incidents that weren't associated with his work.

If you can't get good answers to those questions, I would pass him up.  It could be a huge liability for you and not many insurance companies would be willing to cover you if you're a homeowner.