The controllers in your environment are upgraded at the cluster level and managed by the NSX Manager. Once an upgrade is available, the controller cluster status under the NSX Managers table will display an upgrade is available. During the upgrade, the NSX controller upgrade will download an upgrade file to be downloaded to each controller node for the upgrade:
From NSX 6.3.3, VMware has changed the underlying operating system for the NSX controller node. This means that an upgrade from NSX 6.3.2 or earlier to NSX 6.3.3 will delete the existing controller nodes one at a time and deploy a new Photon OS-based NSX controller node. ...
