A change of clothes; the act of changing clothes, or the clothes themselves that one changes into (e.g., for travel or exercise).
着替えを忘れないで持ってきてください。
Please don't forget to bring a change of clothes.
赤ちゃんの着替えをバッグに入れておいた。
I put the baby's spare clothes in the bag.
旅行中は着替えを二日分用意した。
I packed two days' worth of clothes for the trip.