As you can see on the official web page, Norton 360 Premier Edition has been replaced by Norton Security with Backup and there isn't an online resource available to download it. If you already have the installation package of Norton 360 Premier Edition but you don't know how to activate it, press the Register option, type the serial key received when you purchased the software and click Activate/Register. If you no longer have the key, you will need to contact the official developers. You can do this using the Feedback menu,
+1 800-721-3934 phone number or the contact information from their website.