build up vs improve

build up and improve both are verbs.

Word	Noun	Adjective	Verb	Adverb
build up	No	No	Yes	No
improve	No	No	Yes	No

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

Other hypernyms of build up include ameliorate, amend, better, meliorate.

build up (verb)	improve (verb)
form or accumulate steadily	get better
enlarge, develop, or increase by degrees or in stages	to make better
change the use of and make available or usable
bolster or strengthen
prepare oneself for a military confrontation