。
She spoke Japanese well.
Literal
She [topic-は] Japanese [object-を] [skillfully-上手に] spoke.
上手に is the adverbial form of the na-adjective 上手 ('skillful, good at'). When 上手 appears as a predicate, the thing one is good at is marked with が (日本語が上手だ); here, in adverbial form, the object is plain を.