Yesterday VMware released both NSX for vSphere 6.3.o as well as the accompanying vCenter 6.5a and ESXi 6.5a.
Before upgrading your NSX environment to 6.3 you would need to upgrade your vCenter and ESXi hosts to 6.5a as describe in the KB 2148841
It seems there was a pretty good reasons for VMware holding out on NSX so long since they introduced a lot of great new features with just a fews standouts listed below:
Links:
http://pubs.vmware.com/Release_Notes/en/nsx/6.3.0/releasenotes_nsx_vsphere_630.html
https://kb.vmware.com/kb/2148841
http://pubs.vmware.com/Release_Notes/en/vsphere/65/vsphere-vcenter-server-650a-release-notes.html