Thank you for your post, and I apologize for the late reply.
If you have already found a solution, then disregard this post.
If you have purchased FileMaker Server with a CD, insert the CD and double-click the installation icon. After entering your Mac user name and password, select Uninstall and proceed from there.
If you cannot launch the Installer, then reboot the server, make sure no other applications are running, and try again.