How should I tell my parents?

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

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

Good luck, I hope everything works out :)

They will be sad.

But if Du 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 Du can.

This is all I can say.

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