Hi John,
You will contact your sales rep regarding your interest in upgrading. You will receive an email From Autodesk with your Serial number and Product Key.
These are the values you need to input to activate Advance Steel 2015.1. Below you will finds a link that will help.
INSTALLATION INFO
http://www.graitec.com/en/FAQ_view.asp?FAQID=1168&Language=EN&Name=How+do+I+activate+my+Autodesk+Advance+Steel+license%3F
INSTALLATION, ACTIVATION &LICENSING
http://knowledge.autodesk.com/customer-service/installation-activation-licensing
I hope this helps.
Best Regards.