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She changed her hairstyle during summer break.
Literal
She [topic-は] summer-vacation [of-の] during [in-に] hairstyle [object-を] changed.
夏休みの間に is the standard 'during summer vacation' frame: N + の間に = 'during N, within the interval of N.' The に is essential — without it, ~の間 would mean 'during/while' with a slightly different emphasis on duration rather than punctual occurrence within. Changing one's hairstyle during a school break is a Japanese cultural minor classic — many strict schools forbid coloring, perming, or dramatic cuts during the school year, so the long break is when young people often experiment with their look.