If it were an upgrade from 15_x to another 15_x more recent, you didn't have to do anything special.
Since it's a previous version (11), you will have to use "sqlupgraderes" tool. That tool will help you with migration. Take a look at the installation guide for ASE 15. There is a section there for upgrades.
The version 15 bring new algorithims for the optimizer, for instance queries with the group by on version 15 are not ordered like before due to the choosen algorithm. There are more details to take in consideration when upgrading.
Also you didn't mencioned the OS that you're using, on the manual choose the one that suits you.
Go to SyBooks Online
and choose "Adaptive server 15" and then the installation guide for your OS.