const [action, setAction] = useState("login"); //login is default value

<StateChanger>
   {action === "login" ? (
    <>
     계정이 없다면
     <Link onClick={() => setAction("signup")}>회원가입</Link> //setAction: change actions`s value
    </>
   ) : (
    <>
     계정이 있다면
     <Link onClick={() => setAction("login")}>로그인</Link>
    </>
  )}
</StateChanger>