sure you can, technically.
please to go https://sort.veritas.com/home
and click on "Installation and Upgrade Checklist" for upgrad related info.
If you are on a 3.x version, you would nopt be able to upgrade to 6.1 directly by running installer.
However, you can always to a package remove then to a "fresh install". Remember do not in anyway change the data on the external storage (dgs/vols with end user/app data).
If you are really on a 3.x version which is very very old, the OS that the 3.x is running on is most likely no longer a supported version, not only by Veritas, also by Microsoft. So an OS upgrade is also required.
Any question? Please do not hesitate to ask.
