Hi,

Your thermostat is located on the right side of the block as you look at it
from the front. It should be under your distributor.

There are two bolts in it I believe, but removal of the thermostat, as far
as I am aware, isn't possible. I think on your Punto it is all one piece
with the housing. Your nearest parts dealer will be able to supply a new
one.

Testing of the thermostat is simple. Immerse it in hot water and see if it
opens. If it does, your problem doesn't lie here.

Take the bottom hose that connects to your radiator off at the thermostat
housing and remove the expansion tank cap (don't do this with a hot engine).
Give it a good squeeze, and ensure no bubbles rise to the surface. This will
remove airlocks.

Best of luck