These three terms, cyborgs, transhumans, posthumans, they are not really very well defined. They're probably causing more confusion than clarity. The cyborg has a reasonably clear definition. It's the idea of combining technology, especially robotic technology, with the human body. Transhuman is meant to be something on its way to our intermediary form between human and posthuman. But there is no good definition of posthuman, so it's obviously not clear either what would count as some intermediate form.
The interesting concept of a posthuman in my mind is one that, who has at least one of their basic pilotical capacities enhanced to a very great degree such that they are, you know, far, have far more of that capability than any existing human. The basic capacities might be a healthy life expectancy, cognitive ability, perhaps emotional, the level of control and the richness of your emotional life or something like that.
So if you could radically increase any of those, and not just by having some different, in the culture, ideology that makes you describe your experiences in different ways, but we're actually changing the human hardware, in a profound way. I think that might deserve to be called posthuman.