Вы можете просто назначить новое свойство.
С классическим назначением:
const
array = [{ club: "Club A", number: 12 }, { club: "Club B", number: 12 }],
member = ["a", "b"];
array.find(({ club }) => club === "Club A").member = member;
console.log(array);
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Или взять Object.assign
const
array = [{ club: "Club A", number: 12 }, { club: "Club B", number: 12 }],
member = ["a", "b"];
Object.assign(array.find(({ club }) => club === "Club A"), { member });
console.log(array);
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