。
Vegetable prices change day by day.
Literal
Vegetable [gen-の] price [topic-は] day-by-day changes.
日々 ('day by day, daily, each day') is a reduplicated form — common in Japanese for creating adverbial/noun forms expressing iterative time ('every X'). Other examples: 年々, 月々, 人々. The dictionary form 変わる appears as habitual-present.