Commultiplication and antipode in Hopf algebra?

Question

Let $H$ be a Hopf algebra with antipode $S$. For $h \in H$, we have $(S \otimes S) \circ \Delta(h) = \tau \circ \Delta \circ S(h)$, where $\tau(a \otimes b) = b \otimes a$ and $\Delta$ is the commultiplication. How to prove this identity? Thank you very much.