You can drag an image from un-named people up on to a stack in named people. But I'm not sure it will stick as the top image.
Thank you for your answer, you are right but what I want is to be able to choose one of them as portrait image of the stack.
LR doesn't let you do that. Others have asked for the same feature -- see this topic in the official Adobe feedback forum: Lightroom 6/CC - Allow Named People thumbnail photo to be changed. Please add your vote and opinion to make it more likely Adobe will prioritize an improvement.