You can review the support bundle log messages to identify the upgrade problem.
You can also perform any of the following debugging tasks.
Log in to the NSX Manager CLI as root user and navigate to the upgrade coordinator log files /var/log/upgrade-coordinator/upgrade-coordinator.log.
Navigate to the system log files /var/log/syslog or API log files /var/log/proton/nsxapi.log.
Configure a remote logging server and send log messages for troubleshooting. See NSX-T Data Center Administration Guide.
If you are unable to troubleshoot the failure and want to revert to the previous working version of NSX-T Data Center, contact VMware support.