。
She's not the person she used to be.
Literal
She [topic-は] past [possessive-の] her is-not.
昔の彼女 means 'the her of the past' — の connects the time reference 昔 to the person 彼女. ではない negates the identification: she is not that person. The sentence captures the idea of personal transformation so completely that one's past self feels like a different person entirely. This is a compact, evocative expression of change.