。
She changed her hairstyle.
Literal
She [topic-は] hairstyle [object-を] changed.
変える is the transitive partner in the transitive/intransitive pair 変える ('to change [something]') / 変わる ('to change [oneself]'). The choice of transitive 変える here puts her in the active role — she made the change happen, rather than her hairstyle changing on its own. ヘアスタイル is a katakana loanword and a single conceptual unit, not parsed by Japanese speakers as 'hair' + 'style' separately.