I would like to simplify the expression
$\left(\phi(s_{1})\cdot(X_{s_{1}}-X_{s_{2}})+\phi(s_{2})\cdot(X_{s_{2}}-X_{s_{3}})+\ldots+\phi(s_{n-1})\cdot(X_{s_{n-1}}-X_{s_{n}})\right)^{2}$
where $X_{t}$ is an Ito process, $\phi\in C^{\infty}$ and $s_{1}\geq s_{2}\ldots\geq s_{n}$. How can I apply Ito formula in order to simplify the above expression? More precisely I would like to get rid of the $^{2}$.