NSX-V v6.3.3 was released this week, but before you upgrade be sure to pay close attention to the release notes.  Couple of things to note.

NSX Controller OS change
The underlying operating system for the NSX Controller is now Photon OS. See Controller Upgrade in the Upgrade Notes for NSX Components section for details on upgrading controllers to NSX 6.3.3

Controller upgrade may take a little longer than usual, as when you upgrade to NSX 6.3.3, instead of an in-place software upgrade, the existing controllers are deleted one at a time, and new Photon OS based controllers are deployed using the same IP addresses.
Also review the new storage requirements, in NSX 6.3.3, the NSX Controller appliance disk size changes from 20GB to 28GB.

So ensure you have enough capacity in your management cluster to cater for the 3x28GB for the controllers and 28GB for the new controllers as they are deployed, before the old controllers are deleted.

Also review this KB 
NSX 6.3.3 does not support ESXi versions earlier than 6.0 Update 2 or 5.5 GA (2151267)

The minimum supported version of ESXi running with NSX 6.3.3 is as follows:

ESXi 5.5 GA
ESXi 6.0 Update 2.
ESXi 6.5a (no change from earlier 6.3.* versions).

If NSX 6.3.3 is installed on an earlier version of 5.5/6.0 ESXi, the netcpa service will fail to start preventing communication between ESXi hosts and the NSX Controllers.

Check the VMware Product Interoperability Matrix and Upgrade Path for complete details

• NSX