。
She partially changed the plan.
Literal
She [topic-は] plan [object-を] partly changed.
一部 functions adverbially here ('partially, in part') without needing the adverbializing に — Japanese routinely uses bare nouns as adverbs when the meaning is quantitative or temporal: 全部 ('all'), 半分 ('half'), 一部 ('partly'), 少し ('a little'). The Sino-Japanese 変更する is the formal counterpart to the native verb 変える: news writing and business documents prefer 変更 while everyday speech uses 変える. Both mean 'to change,' but the registers differ sharply.