As per SORT (https://sort.veritas.com/eosl), NetBackup 7.6 went out of support on 2017-01-31. So, you/your client is on their own for the upgrade.
It will have to be a multiple step upgrade, considering that Windows 2008 R2 is not supported and that all versions till NetBackup 8.0 are EOSL currently.
Check the SCL checker https://sort.veritas.com/nbu_checkers/nbu_scl_checker. It shows that 7.7.x versions are supported on Windows 2008 R2. So first upgrade NetBackup to 7.7.3, then go for OS upgrade to 2012 R2 (you will need to choose the way to upgrade the OS - I suggest, get new hardware, install new OS, use your method to "migrate" NetBackup and data to new hardware or use Catalog recovery as pats_729 mentioned). Once you are on 7.7.3/Windows 2012 R2, upgrade NetBackup to whichever version that you require (don't forget that security certificates will be required from version 8.1). Then plan to upgrade OS to 2016 or 2019.
PS: 8.3 goes EOSL in July 2024, best to upgrade to 9.x.