lighten up vs modify

lighten up and modify both are verbs.

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

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

Other hypernyms of lighten up include alter, change.

lighten up (verb)	modify (verb)
make more cheerful through the use of color	make less severe or harsh or extreme
become lighter	add a modifier to a constituent
become more cheerful	cause to change; make different; cause a transformation
make more cheerful