You would use qué instead of cómo, "qué es hoy?". Qué means 'what'.
Duolingo sort of misleads us early learners into thinking cómo means 'what' because of phrases like 'cómo te llamas?' (what is your name), but that phrase actually means something like "how are you called? ... at least in my rough understanding.
Literally, this says "How you-are today?". The verb tells us that the subject is "you" even thouh the "tu" is missing (as is normal in Spanish). So here, as so often, the verb is the big clue as to the meaning.
To translate your version, the verb would need to be "esta" ir "it is".