。
She transferred from the bus to the subway.
Literal
She [topic-は] bus [from-から] subway [to-に] transferred.
乗り換える is a compound verb literally 'ride-change,' the standard word for transferring between trains, buses, or subway lines. The frame [departure]から[arrival]に乗り換える is fixed: から marks where you're getting off, に marks where you're boarding next. Multi-modal transfers like this are routine in Japanese cities, where 乗換案内 ('transfer guide') apps map even minor stations and timetables exhaustively, and major hubs like Shinjuku integrate buses, subways, JR lines, and private rail under one roof.