Hi,
you have to upgrade the environment (vCenter Server + ESXi hosts) to software version 5.1. The license key will stay the same for all 5.X versions.
Here is a link to the vSphere 5.1 Upgrade Guide: http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-51-upgrade-guide.pdf
Regards