decrease vs alter

decrease is a noun but alter is not a noun.

Word	Noun	Adjective	Verb	Adverb
decrease	Yes	No	Yes	No
alter	No	No	Yes	No

As verbs, alter is a hypernym of decrease; that is, alter is a word with a broader meaning than decrease:

Other hypernyms of decrease include change, modify.

decrease (noun)	alter (noun)
the act of decreasing or reducing something
the amount by which something decreases
a change downward
a process of becoming smaller or shorter

decrease (verb)	alter (verb)
decrease in size, extent, or range	remove the ovaries of
make become smaller	become different in some particular way, without permanently losing one's or its former characteristics or essence
	cause to change; make different; cause a transformation
	insert words into texts, often falsifying it thereby
	make an alteration to