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She has to change clothes before tonight's party.
Literal
She [topic-は] tonight 's party 's [before-前に], outfit-change [object-を] do [must-なければならない].
着替えをする is built from the verb 着替える ('to change clothes') turned into a noun 着替え via the masu-stem nominalization. Pairing it with する is a roundabout way of saying the same thing as plain 着替える, but the [noun]+を+する frame is extremely productive — 勉強をする, 買い物をする, 散歩をする — and Japanese often prefers it for stylistic balance or emphasis on the activity as a discrete event. ~なければならない is the textbook 'must' pattern; in casual speech it shrinks to ~なきゃ or ~なくちゃ.