If you want to upgrade NetBackup from 7.5 to 7.6, you should follow the link regarding to NetBackup 7.6
RTFM is often used as a response to technical questions. Unfortunately, some folks do not even know how to find the manual, let alone understand it.
Yesterday, EMC announced updates to it's Data Domain documentation. Use the links to view the compatibility guides.
How to isolate the cause for the issue and identify the solution whilst connecting to the data domain system over a network.