Hi,
Sylink.xml location has changed in SEP 12.1
Click on Start
Click Run
Type smc -stop in the run field
Copy the sylink.xml file and paste under "C:\Documents and Settings\All Users\Application Data\Symantec\Symantec Endpoint Protection\12.1.xx.xx\Data\Config" it will prompt do you want to replace the file click on Yes.
Start the smc service
Click Start
Click Run
In the run field type the following command
smc -start
If you want to replace it through utility then as Paul mentioned you can do it with the help of Sylinkdrop utility
You can find it under tools folder with original setup files.