Don't even try - you'll probably break the screen.
It is held in by a tough silicon adhesive that needs a special shaped knife to cut round it and break the seal and even then, the rubber seal can be damaged and you can't put the screen back.
Do the sensible thing - take it to your local windscreen place and get them to cut it our for you - they'll only charge you a couple of quid to do it.
When replacing the screen you'll need to use the same silicon bonding agent to put it back to secure it - you CANNOT use ordinary silicon