Please do check below points
1. A Supported browser (Clear the web browser cache and delete offline stored objects.)
2. Do check filesystem usage in NSX Manager (show filesystems) .If filesystem usage is high we might need a purge task to clean up.
3. NSX Manager upgrade bundle MD5 should be validated( you can use any third part tool for this)
4. If everything is fine, try an api call for the upgrade. (https://docs.vmware.com/en/VMware-NSX-Data-Center-for-vSphere/6.3/nsx_63_api.pdf page 128)
What browser are you using? I had the same problem and it worked after I tried from a different browser (tried chrome first, firefox second). But it could also have been the browser cache that was the problem as suggested above.
I had the same issue. After much troubleshooting, I had to resort to the API upgrade method which worked. I then did the other 5 NSX Manager upgrades the same way via API.