OK, I found the solution. It was a few links down. . .
https://www-secure.symantec.com/connect/forums/backup-exec-appliance-3600-r2-upgrade-last-version-be2012-sp2-or-sp3
I've got a Version Upgrade Notification for the 3600 appliance to Backup Exec 2014, download the .zip file of the ISO and get the license file. Where can I find information about the upgrade process for the 3600 appliance ?
Thanks in advance