You can't change people because of how they act differently, only the person herself/himself can make that change but people change over time especially when they've been in your friend circle or life. You can't make people be what you want them to be or change them for who they really are if they're trying to fit in or they want to talk to you. You don't need to do so. If they're acting differently to towards you and they not the person they used to be then maybe it's time to move on and accept that the person's behavior has changed.