When a patch is released for Final Draft, you will receive a notice indicating an update is available. However, if you dismiss this notice or don't receive it, you can always update Final Draft by going to Help in the menu bar and clicking Check For Updates:
Please note that this will not update Final Draft to a new numbered version, Final Draft 12 won't update to Final Draft 13, etc. This is only for patches for individual numbered versions, like going from FD13.2.4 to FD13.2.5.