so there's no set rule. But ZA will install on top of itself, removing the old version automatically.
I think I've read when there is an update to a program you're using that you should uninstall what you're running and then install the update. And I think I've read as well you can install the update on top of what you're already running. I've also seen a program uninstall itself before installing it's update. Any rule to go by? Zone Alarm (free version) has come out with an update and it would be so much easier to install on top of the other.