Communication is key in pick and roll defense. Now, if you happen to know a screen is coming, press up on the ball handler so you can avoid it. But if you do end up getting screened, you have two options.
Depending on how hard the screen hits, either go over it or switch with your teammate.
If you do get screened hard, you must immediately communicate with your teammate that you need the switch. Be sure to be loud enough for them to hear you.
If you decide to go around the screen, never go under, always go over. Going under allows too much time and space for the shooter to get their shot off.