vCenter Server versions 5.1 and later require the vCenter Single Sign-On and vCenter Inventory Service components. Depending on your existing vCenter Server installation, you can use the Simple Install option to upgrade to vCenter Server, including vCenter Single Sign-On, the vSphere Web Client, and Inventory Service, all on a single host machine.
You can use Simple Install to upgrade vCenter Server if you have a version 4.x, 5.0.x, or 5.1.x vCenter Server installation that is supported for upgrade, and all vCenter Server components in the installation you are upgrading are on the same host machine. See the VMware Product Interoperability Matrix at http://www.vmware.com/resources/compatibility/sim/interop_matrix.php. Depending on the version you are upgrading from, the Simple Install option installs or upgrades Single Sign-On, and upgrades the vSphere Web Client, Inventory Service, and vCenter Server.
Alternatively, you can upgrade vCenter Server components separately, for installations in which the location and configuration of the components is customized. See Use Custom Install to Upgrade a Standalone vCenter Single Sign-On Deployment of Version 5.1.x vCenter Server and Required Components, Use Custom Install to Upgrade vCenter Server from a Version 5.1.x High Availability vCenter Single Sign-On Deployment, or Use Custom Install to Upgrade vCenter Server from a Version 5.1.x Multisite vCenter Single Sign-On Deployment.