[Zope] Zope upgrading

Hi, I'm a newbie. I' ve this problem. I've a Zope 2.7.2 installation on a windows machine, a zope instance with some products installed (ZODBC etc.) and an application. I have to upgrade zope to 2.7.3: how can I install the new version of Zope? I have to uninstall the old version before installing the new? How can I keep my previous instance? I've found some indications on the web and in this list, but they aren't so clear to me. Please help me!! Piergiorgio