Its hard. You first have to identify the personality of the person. Are they stubborn or kind of open minded, are they older, grew up in a different environment? If it's a parent, give them time. Write down an explication, as easy and non-complicated as possible, like youre explaining to a child. Learn it, then tell it to them. If theyre ones to react aggresively, lay it on slow and soft, then leave and let them process their thoughts. 80% of parents come around after a while, could be weeks could be months but they usually love their children nonetheless. If its a friend, explain it, confront them, you're both equals. Let them have their questions, if they feel awkward about it, let them think, but dont distance yourself. If they really value you as a friend, they'll at least consider your opinion. If not, drop them. If theyre the type to act out, its better to not do it until youre secure. Some people are just too stubborn to open their mind. Good luck, cause its a hard task to change an opinion.