It depends on the operating system. You can do it manually, but the uninstall is the best way to do it. Just stop the service with Admin Console before uninstalling it. I assume you don't have any databases you need on there.
By chance I ran the uninstaller for Server 13... then ran installer for server 13 again and successfully installed.... Thinking it uninstall Server 10.
Ahhh.... surprising. I did not realize Server 13 would uninstall previous versions. Good to know!
/Library/Application Support/FileMaker/FileMaker Server/FMS 10 Uninstaller.app
or not installed?
Similarly, I did not realize Server 13 would uninstall previous versions.