。
Yumi speaks English very well.
Literal
Yumi [topic-は] English [object-を] very well speak.
上手に ('skillfully, well') is the adverbial form of the na-adjective 上手 ('skillful, good at'). Distinct from 上手 used predicatively as 上手です ('is skilled at X'), where the thing one is skilled at is marked with が. Here the adverbial use modifies 話す directly.