using product key from third party.......

I thought I opened up a new thread but can't seem to find it.  Sorry if this is a double post....... 

Trying to find out how to use a product key for Norton Security (5 devices) downloaded from Amazon to "update" an installation of 360 on 3 systems.  I have not been able to find a how-to and really would like to make sure I do not blunder trying to get Norton  Security working.  Is there a written tutorial I can follow somewhere?  I have looked but not been able to find anything other than some comments on Amazon about how confusing the process is.  I have ~3 days left on 360 but would rather not wait until the last minute.  

Do I treat this as a complete new install and delete 360 first or can I "update" using Norton Security?   I'm really at sea here and need help.....

Thanks very much for any and all help.......