lessen vs alter

lessen and alter both are verbs.

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

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

Other hypernyms of lessen include change, modify.

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