How should I tell my parents?

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

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

Good luck, I hope everything works out :)

They will be sad.

But if wewe 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 wewe can.

This is all I can say.

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