call the support and ask for a downgrade
(sorry for the short answer - forum software crashes all the time)
Did you save your installer for FileMaker 12? if so, you need only re-install. (Always save multiple back up copies of your installer, you never know when a computer might "die" and you'll need to install on a new machine.)
@PhilModJunk: Not getting you . Could please elaborate lil bit more ??
When you purchased FileMaker 12, it came as an installer--either on disk or via download. If you still have that file, you can simply run it again to install the software back on your computer. You can even keep 12 and 13 installed on the same machine. If you did not keep the installer, you can request a new copy of the FM 12 installer from customer service.
Since you never know when a computer might suffer a sudden, catastrophic failure, it's always a good idea to keep a copy of this installer on a flash drive or something so that you can install on new hardware in cases of such an emergency.