Before you have a leg to stand on, you must first prove that the person who changed the will was mentally incompetent and unaware of what he was doing.
If he did it and then died 4-6 months afterwards, you might could prove that on your own. If it has been years since he died, it is going to be hard, if not even impossible. Best bet is to first talk with the physician that treated the individual or his family. If they agree he wasn't in his right mind or thinking rationally, then you can get an attorney to proceed.