How does Atticus defend Calpurnia from Aunt Alexandra?

I assume that you are talking about what happens in
Chapter 14.  There, Atticus tells Aunt Alexandra a number of
things.

First, he says that Calpurnia has done a good job
raising the kids.  He says

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If anything, she's been harder on
them in some ways than a mother would have been... she's never let them get away with
anything, she's never indulged them the way most colored nurses
do.

He also
tells Alexandra that the children love Calpurnia for how she has raised
them.

He tells Aunt Alexandra that he will never throw
Calpurnia out because she is essentially a member of the
family

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"Alexandra, Calpurnia's not leaving
this house until she wants to. You may think otherwise, but I couldn't have got along
without her all these years. She's a faithful member of this family...

By
saying these things, he is clearly defending Calpurnia against Aunt Alexandra, who has
come in part because she does not approve of a black woman raising Scout as she gets
older.