How should I tell my parents?

Your beliefs are your beliefs. They have no right to make you feel bad about them. You have every right to think what you want.
Your parents should accept you as an atheist.

I'd try and be gentle about it....pick a good time to talk to them, and tell them that you respect their faith, and that you hope they'll respect yours.

Good luck, I hope everything works out :)

They will be sad.

But if you are honest and true in your beliefs, there is nothing they can do, but try.

Just be yourself around them, speak your mind and try to be as subtle as you can.

This is all I can say.

Just tell them. They'll except you they're your parents, it an instinct