For example, if you want to upgrade NetBackup from 7.5 to 7.6, you should follow the link regarding to NetBackup 7.6.
In the above documentations links, you can find not only the Upgrade Guide/Release Notes for a specific NetBackup version but also how to configure Exchange, Sharepoint backup and more as well.
It is useful when a newer NetBackup Client software does not support specific client OS. NetBackup supports a mixed version environment - it means that you can upgrade the OpsCenter, Master, Media Servers and leave some clients with older NetBackup software installed.