mending vs improvement

mending and improvement both are nouns.

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

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

mending (noun)	improvement (noun)
the act of putting something in working order again	the act of improving something
garments that must be repaired	a change for the better; progress in development
	a condition superior to an earlier condition