How are you?

Hola Valerie L.

Using someone's first name doesn't necessarily mean that you would address them informally. You can know someone's first name and still use usted because of the relationship, the situation, age or simply personal/regional preference.

So, for example, “María, ¿cómo está?” is perfectly natural when addressing María as usted, while “María, ¿cómo estás?” would be used when addressing her as tú.

I hope this helps!

Silvia