Reboot the host and press R to start the Recovery Mode.
Press Y to start the roll back
The host will downgraded and back online again with VMware vSphere ESXi 5.0.0
Note: You cannot downgrade vCenter once updated.
i was able to sucessfully rollback to 5.0 vcenter. diabled all HA on the cluster and renabled them after. The 5.0 agents will upload back into the esxi hosts.
this upgrade is the worst ever. Maybe a brand new setup would haqve been better but the inplace upgrade is very very buggy. even on the newly release 5.1.0a