From 6.5 Install Guide (not GA yet)
Requirements for upgrades to NetBackup 6.5
NetBackup version 6.5 includes many new features. Some new upgrade
requirements have been put in place to make your transition to this version of
NetBackup easier.
The following describes the requirements for NetBackup server upgrades:
■ Always back up your NetBackup databases before upgrading NetBackup
software on the master server.
■ You can upgrade to NetBackup 6.5 only if you have one of the following
versions currently installed:
■ Version 6.0
The procedure for upgrading from 6.0 to 6.5 is included in this
installation guide.
■ Version 5.0MP4 (or later) or version 5.1
Before you can upgrade from these earlier versions, you must first
prepare your current NetBackup environment for an upgrade to
version 6.5.
Though it is recommended to be on the highest MP level but it is not necessary to upgrade from 5.1 MPx to 5.1 MP6 or any other higher MP release if you are have 5.x install. If your NBU version is 5.0 then you must upgrade to MP4 before upgrading to 6.x.
There is no need to upgrade your enviornment to 6.0 MP4 before upgrading to 6.5.
Upgrade process for 6.5 is same as for 6.0.