The latest available version of vCenter Server is Update 1a (VMware-VIMSetup-all-5.1.0-1123966.iso). For update instructions and requirements, please refer to the vSphere Upgrade Guide at http://www.vmware.com/support/pubs/vsphere-esxi-vcenter-server-pubs.html
You may also want to check your hardware for compatibility (http://www.vmware.com/go/hcl) and take a look at the VMware Product Interoperability Matrix.
André