After the ESXi upgrade is failed or Hung, reboot your ESXi host. Durig ESXi host boot up, hit Shift+R
Recovery mode will display the ESXi build version which you were upgrading from. If you want to roll back to the previous version, hit Y to go back to the previous version of ESXi. In my case , I was upgrading my ESXi host from ESXI 5.5 to ESXi 6.0. So I am hitting Y to rollback to previous version ie. ESXi5.5.
Thats it. Your ESXi server will boot with the previous version of the ESXi i.e ESXi 5.5. This is a permanent and you will not able to boot to ESXi 6.0 until your next successful upgrade.