Try to stop SEP service called Symantec Endpoint Protection then try to replace sylink.xml it should help.
BTW check your own permissions to file if everything is ok and you should have ability to replace the file and still can't do this try to stop SEP service then change it to disable and then restart computer (in my case it helps serveral times.)
You can use SEP user interface. Just open GUI of Symantec Endpoint Protection client and click HELP then chose Troubleshooting.
Next, in Communication Settings just import XML file (but check if your client is self managed or no)