You meet your professor in the morning and ask him/her how he/she is.
Buen día, ¿cómo está?
You meet Miss Rojas in the afternoon and ask her how it’s going for her.
Buenas tardes, ¿cómo está?
You greet a good friend and ask him/her how it’s going.
Hola, ¿Cómo estás?