If you are getting a dynamic address from your ISP, you can't change it. You are at the mercy of whatever their DCHP server settings are. BTW - there is no need to change it (that I can think of anyway).
When you press renew, it is looking at your MAC address, seeing you already have/had a lease and just giving it right back to you.
<i>It's always the one thing you never suspected.</i>