It depends... if the issue is subjective then people are entitled to an opinion and there's nothing wrong with them being unwilling to change. Some people like Trump, others despise him. It's just an opinion.. leave it alone.
However, in a case such as this, if you are dealing with a truly open minded and honest individual, you can try by breaking it down into small pieces that can be researched and debated to a definitive conclusion. Take the Luminol samples issue. You can scientifically prove there is great uncertainty, based on the tests performed, that the samples were made from blood, let alone human blood. But even more importantly, there is no disputing that without a DNA profile you can not prove the sample, what ever it is, came from the victim. The science and the rules of forensic investigations dictate the Luminol samples are unreliable and prove nothing.
The key to this is you must be dealing with someone who is open minded, honest and capable of critical thought. They must be capable of changing their opinion if the facts dictate they should. The remaining PGP that I know of don't appear to fit this description. They reason, as Massei and Nencini did, that the samples must have been made from Meredith's blood because "what else could they be" and because it fits their theory. People that reason that way are not going to change their opinion. Why this happened with two courts in Italy I have no idea. But for the general PGP population I suspect it's just a matter of the vast majority of early material on the case, be it the media, websites, police briefings or books published early on all cemented the idea of guilt and that has proven to be unshakable.
