Antonyms are words with opposite meanings. Some antonyms for the phrase "jazzing up" could include words like simplifying, downgrading, or toning down. When you simplify something, you make it less complex and more straightforward. You might do this to make something more accessible or to reduce unnecessary details. Downgrading, on the other hand, implies lowering the quality or importance of something. This could be intentional or unintentional, but the result is that something becomes less impressive or valuable. Toning down is another antonym for jazzing up, which means to make something less bright, flashy, or bold. You might do this to make something more muted or understated.