What are the similarities and differences? To no avail, I tried ODO and this Wordreference.com post.
{verb} 1[.0]. Make or become better:
1.2. [no object] (improve on/upon) Achieve or produce something better than:
Footnote: I encountered this issue while reading Principles of Microeconomics, 7 Ed, 2014, by NG Mankiw, who applies Defn 1.0 on p 11:
Governments Can Sometimes Improve Market Outcomes.
But arrestingly, on p13, Mankiw instead applies Defn 1.2:
To say that the government CAN improve on market outcomes … does not
mean that it always WILL.
Best Answer
Improve: make it better. Improve upon: make something that is better than it.
P.S. Some examples:
We have improved our Comfort Ride toilet by adding a heated seat.
We have improved upon basic toilet design by using mains pressure to assist the flush, moving the same amount of waste with less water.
P.S. When we improve something, me make it incrementally better. When we add the preposition upon (improve upon something) we are casting the object of the preposition as a starting place, a basis, a status quo, the current paradigm. To "improve upon" that paradigm is to make a significant or major advance in respect to it, to "leapfrog" it. You do or make something better than it.