When using systemd I found that I needed to create the config file in the users scripts directory. In fedora 20 that was
and to control the script you then need to use
systemctl --user start syncthing
If you want to use the built in upgrade capabilities you will need to stop the service
systemctl --user stop syncthing
and then run syncthing as root
Then perform the upgrade, shut it down and restart the service.
At least this was the only way I could do it as I ran into permission issues as the service obviously runs under your account and by default may not have access to update the binaries.