。
She bandaged up my injured finger.
Literal
She [topic-は] my injured finger [to-に] bandage [object-を] did.
けがした is the past-tense ~した form of けがする ('to get injured') — used here as a relative-clause modifier of 指 ('finger'), giving 'a finger that got injured.' 包帯をする ('to bandage') treats the bandage as something done — Japanese verbalizes many noun-objects this way: マスクをする (wear a mask), ネクタイをする (wear a tie), シートベルトをする (fasten a seatbelt). The に marks the finger as the recipient of the bandage. This is everyday first-aid vocabulary worth knowing.