lighten up vs alter

lighten up and alter both are verbs.

Word	Noun	Adjective	Verb	Adverb
lighten up	No	No	Yes	No
alter	No	No	Yes	No

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

Other hypernyms of lighten up include change, modify.

lighten up (verb)	alter (verb)
make more cheerful through the use of color	remove the ovaries of
become lighter	become different in some particular way, without permanently losing one's or its former characteristics or essence
become more cheerful	cause to change; make different; cause a transformation
make more cheerful	insert words into texts, often falsifying it thereby
	make an alteration to