upgrade vs improve

upgrade is a noun but improve is not a noun.

Word	Noun	Adjective	Verb	Adverb
upgrade	Yes	No	Yes	No
improve	No	No	Yes	No

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

Other hypernyms of upgrade include ameliorate, amend, better, meliorate.

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

Other hypernyms of upgrade include ameliorate, amend, better, meliorate.

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

Other hypernyms of upgrade include ameliorate, better, meliorate.

upgrade (noun)	improve (noun)
the act of improving something (especially machinery) by raising it to a higher grade (as by adding or replacing components)
hardware that provides better performance than an earlier version did
the property possessed by a slope or surface that rises
a reservation that is improved
software that provides better performance than an earlier version did
an upward slope or grade (as in a road)

upgrade (verb)	improve (verb)
to improve what was old or outdated	get better
give better travel conditions to	to make better
get better travel conditions
rate higher; raise in value or esteem
give a promotion to or assign to a higher position