fixing vs improvement

fixing and improvement both are nouns.

Word	Noun	Adjective	Verb	Adverb
fixing	Yes	No	No	No
improvement	Yes	No	No	No

As nouns, improvement is a hypernym of fixing; that is, improvement is a word with a broader meaning than fixing:

fixing (noun)	improvement (noun)
the act of putting something in working order again	the act of improving something
the sterilization of an animal	a change for the better; progress in development
(histology) the preservation and hardening of a tissue sample to retain as nearly as possible the same relations they had in the living body	a condition superior to an earlier condition
restraint that attaches to something or holds something in place