VMware vCenter Server 5.0 Update 2 Release Notes
Looks like the VMware Download portal is also reporting the wrong version. Release Notes capture the correct build numbers.
vCenter Server 5.0 Update 2 | 20 DEC 2012 | Build 913577
vSphere Client 5.0 Update 2 | 20 DEC 2012 | Build 913577
vCenter Server 5.0 Update 2 Installation Package | 20 DEC 2012 | Build 923238
The 923238 is the build number of the installation ISO. 913577 is what you are on and already the latest build number for vCenter Server itself. That is why you do not get an update option.