You may want to choose the name of your instance wisely at the beginning of the registration, as it cannot be changed later.
The instance name also defines the system e-mail address and the "from" of your outgoing e-mails.
When you create your Zammad instance you will receive your individual Zammad email address, which you can use for your tests or even for "real" customer messages. However, if you want to use the same addresses as before - without your customers noticing that you have changed the system - no problem. Our documentation explains in detail how to connect your own inboxes.
However, if you want to change the URL of your system, we can do this for you. We explain how this is done here.