I'm not sure about the new Punto but on my Mk 1 the whole rear light cluster comes away by removing a couple of bolts on the boot pillars. The new Punto still has high lighting of course so I would guess that bolt is still there, probably hiding behind a flap and/or some boot upholstery. The Mk 1 comes with a plastic tool in it's spare wheel tool kit which enables you to undo said bolt (mind you it's so weak I used a socket set to do it) but I honestly haven;t got a clue if that's still how it's done. That bolt's got to be there somewhere though I'm sure, because they do still have high pillar lighting.
Assuming you get there, once the cluster is loose be careful not to drop it! Unplug the wiring connectors and take the cluster inside or somewhere light to change the bulb. I think I'm right in saying it's the lowest light on the latest Puntos? It should just be a standard twist out, twist new one in jobbie. Then replace the whole cluster and tighten her up. Remember, I'm just assuming that the same basic principal has remained true for the latest Punto's, but it does seem likely.
Also, this should be in your handbook I'd have thought.
Spirito di Punto