。
She turned the picture upside down.
Literal
She [topic-は] that picture [object-を] up-down upside-down-into changed.
上下さかさま is a redundant-feeling but very common compound: 上下 ('up-down') and さかさま ('upside-down / reversed') reinforce each other for emphasis. Japanese is often comfortable with such repetitive amplification (二度と再び 'never again' is another). The に here marks the result of the change: かえる ('to change') with に specifies what something becomes — '[she] changed [the picture] to upside-down'. Note the kanji ambiguity in かえる: 変える ('change'), 替える ('replace'), 換える ('exchange') — all read かえる, with the choice depending on nuance.