Backup Exec Upgrade 2012 to 2015

Wondering if someone could walk me through the backup process from backup exec 2012 to 2015. I have license keys for both 2014 and 2015 which I've heard you need to upgrade to 2014 and then to 2015 from 2012.. Is this correct?? Also where do I apply the license keys for the upgrade. I'm new to upgrading backup exec so a step by step guide would be awesome I just can't find one.

Thanks in advance