If you installed Syncthing with a package manager (such as Homebrew, apt/dpkg, yum, etc), then the packagers should have disabled the auto-update mechanism. You should update Syncthing with Homebrew:
Restarting from the web ui after the upgrade will probably work but this is also worth mentioning:
==> Caveats
To reload syncthing after an upgrade:
launchctl unload ~/Library/LaunchAgents/homebrew.mxcl.syncthing.plist
launchctl load ~/Library/LaunchAgents/homebrew.mxcl.syncthing.plist
Why are you even using the “package manager” version which will not auto-upgrade. I have put time and effort to create a nice installer for OSX and App from the official signed releases. This also includes file-change notification which speeds up syncing. See here: https://github.com/xor-gate/syncthing-mac/releases if you don’t trust it you could verify the executables checksums yourself which are packed in the .app.
It is written in the documentation, currently the Syncthing developers dont care to much at good installers and distributing the executables in a user friendly way so this is “a community effort”: http://docs.syncthing.net/users/contrib.html#os-x