1 Backup your vCenter database.
2 Uninstall all vSphere client and plugins. If VUM plugin cannot be uninstall successfully, check this.
3 Upgrade vCenter / VUM software.
4 If you have changed your database before upgrading, vCenter installer maybe setup a SqlServer 2008R2 Express but never used. You can follow this step to change the service dependence and uninstall this instance of SqlServer. Warning: Make sure the instance is truly useless before you uninstall it.
5 Install a vSphere client and a VUM plugin.
6 Open vSphere client, go to Home – Solutions and Applications – Update Manager, ESXi Images tab.
7 Click Import ESXi Image, upload the ESXi 5.0 installation ISO file.
8 Go to Baselines and Groups tab, create a new baseline for hosts with type Host Upgrade.
9 Go to Home – Inventory – Hosts and Clusters, select the cluster or esxi hypervisor, Update Manager tab.
10 Click Attach, to link your selected hypervisors with new created baseline.
12 Put some of hypervisors into maintenance mode.
13 Important: go to Conguration tab – Software – Security Profile, disable the Lockdown Mode. Or, HA agants will be unable to upgrade.
14 Remediate (context menu item of hypervisor) with Upgrade baseline, select the remove 3rd drivers checkbox.
15 The progress will stay at 22% for more than 5 minutes. You will be notified to press Enter to restart by hypervisor console, not vSphere client. After you press Enter, the CDROM tray will be ejected. You have to close it manually. That is funny, right?
16 Exit maintenance mode.
17 Re-enable the Lockdown Mode.
18 Upgrade license if you need.
19 Upgrade all vm-tools.
20 Upgrade distributed network switches.
21 Upgrade VMFS (terrible). I suggest you to delete and re-create the datastore one by one, by using Storage vMotion moving. It will cost several days.