。
She's good at changing the subject.
Literal
She [topic-は] topic [object-を] changing [subject-が] skillful.
Uses the nominalizer の to turn 話題を変える into a noun phrase, then marks it with が as the subject of うまい ('skillful'). This のが pattern is the standard way to describe what someone is good or bad at. うまい is more casual than 上手 (じょうず) and often implies a natural knack rather than learned skill.