First of all make sure you backup all of your data and that you verify that you can restore and use it.
Next uninstall 12.5 and make sure to remove all the data. If you do not this will cause more problems for you shortly.
Find your install copy of 11D and install it.
Once your installation is complete stop the backup exec 11d service and copy all your data back from 12.5 to it’s original location (Awesome that the paths have not changed).
Now you want to restart the services.
On any of the clients that you have upgraded you will need to install the old 11d clients otherwise they will not be able to connect.
Before you start AGAIN make sure you have a FULL BACKUP and can access the data!
**Keep in mind that the newer agents already installed on client servers will have to be completely uninstalled, so there is no possibility that v.12.5 files are detectable when installing 11D.