There isn't a way, uninstall filemaker and reinstall it. Unless you like hacking the registry to bits, which would take longer anyways, just do a reinstall.
I also recommend using something like REVO uninstaller, which deletes all traces of the program before you reinstall. This can help get rid of errant registry keys and temp files.
oh, and if you're worried about getting the installer software, filemaker generates an email with your license key (and the downgrade) that gives you a download link, good for 30 days, where you can download the FM11 installer.