let up vs lessen

let up and lessen both are verbs.

Word	Noun	Adjective	Verb	Adverb
let up	No	No	Yes	No
lessen	No	No	Yes	No

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

Other hypernyms of let up include decrease, diminish, fall.

let up (verb)	lessen (verb)
become less in amount or intensity	decrease in size, extent, or range
reduce pressure or intensity	wear off or die down
	make become smaller