1 general:: Make beneficial additions or changes to, as in The company is trying to improve on the previous model. [Late 1600s]
American Heritage Idioms
2 general:: Phrase(s): improve (up)on something to make something better. (Upon is formal and less commonly used than on.) • Do you really think you can improve upon this song? • No one can improve on my favorite melody.
McGrawhill's American Idioms And Phrasal Verbs