Consider this question and its related answer:
How was the pizza?
It was delicious.
The question is asking how, which is defined in every dictionary as an adverb, but the answer is delicious, which is an adjective.
Why is how considered an adverb, even in this case?
The Oxford English Dictionary has a very long page of meanings for how (two senses are quoted below).
Probably what you need is a better quality dictionary/grammar book!
The “adverb meaning”
I.1.a. Qualifying a verb: In what way or manner? By what means?
The “adjective” meaning.
I.2.a. In what condition or state? how are you?: (in quot. 1918) used ironically in sense ‘indeed!’ how do you do? (formerly how do you?):
common phrases used in inquiring as to a person’s health. See also
how-do-ye phr. and n., how-do-you-do phr. and n. Also, how goes it?
= how-do-you-do phr. and n. 1; how’s (or how are) things (or, orig. Austral. and N.Z., tricks?) ; how do?: = how-do-you-do phr. and n.