To access the lower bolts on the headlight, you'll need to remove the bumper. There is an excellent YouTube video from PuntoHowTo on removing the bumper as well as a guide on the forum; I've linked both below. Once the bumpers off, you'll be able to remove the headlight without issue. Just be sure to take note of the alignment so the gaps are all equal.
Edit: I do believe you can leave the bumper attached and instead detach the bumper from around the arch/wing area to access the bolt however I'm not sure on that. Have seen it done on some cars but never tried it on a Punto.