You can basically follow http://kb.vmware.com/kb/2008939 for a standalone ESXi host. In case you have Update Manager (bundles with vCenter Server) in place, this might be the first choice for patching. In any case make sure your hardware is supported with the patch/update you are going to install (see http://www.vmware.com/go/hcl) and consider to backup the VMs just in case something unexpected happens.
André