Hi Paolo & Bienvenuto a te ;-)
The upgrade from one release to another (7 to 8) is not available yet,
Trisquel 8 is still Alpha, nevertheless you can install Lxde (even Lxqt),
Xfce,Cinnamon with your Synaptic Package Manager.
RazorQt is the second desktop environment by default in the Mate version (by
selecting at the login with the Trisquel logo) you will have to add
razor-panel to make it more functional.
I had no success with the Netinstall images either..