The phrase "improved upon" is a common way of indicating that something has been made better. However, there are a variety of synonyms that can be used in its place to add variety and precision to your writing. Some alternatives include "refined," "enhanced," "upgraded," "elevated," "transformed," "perfected," "amplified," "advanced," "boosted," "augmented," "polished," "elevated," and "strengthened." Each of these words conveys a slightly different nuance, so choosing the right one will depend on the specific context and desired tone of your writing. Whether you opt for a common synonym or a more unusual one, experimenting with varied language will make your writing more engaging and dynamic.