There's difference between update/patch and upgrade. For an Upgrade (i.e. v5.0 to v5.1) you first need to upgrade vCenter Server as well as Update Manager to version 5.1, because vCenter Server 5.0 doesn't support ESXi 5.1 hosts. Once vCenter Server and Update Manager are both upgraded, you can create a "Host Upgrade" baseline to upgrade the hosts. Make sure your hardware is supported prior to upgrading.
Btw. always make sure vCenter Server and the hosts are running ina supported configuration (see VMware Product Interoperability Matrix)
André