Well, thats a very good question, cause each time someone asks the answer is usually that noone really knows.
As far as I know, there is no menu in WZ to manage your membership.
If i remember correctly, you can cancel the auto-renewing membership from within Paypal, and buy a different member afterwards.
Considering that you're paying for it with real money, you might want to reach out to Fizzer (
https://www.warzone.com/Contact) to get a confirmed answer on how to deal with this.
(If you find out anything new from your chat with Fizzer, please share it with us so we know as well for next time!)
