You have two choices:
How do you say it?
What do you say?
In the first sentence, you want to know the method. (How do you say ______?)
For example, "How do you say 'computer' in French?" or "How do you pronounce 'can' in English?"
In the second sentence, you want to know the thing that you say.
For example, "What do you say when you bump into someone by accident?" You say, "Excuse me."
We also use "What do you say?" when we want someone's opinion. "Let's eat sushi. What do you say?"