Yes you can not edit the alarms that are configured on the root level (vCenter level) on the Datacenter level.
As per the vSphere 5.5 Documentation Center:
"To monitor your environment, you can create and modify alarm definitions in the vSphere Web Client. You can view alarm settings from any object, but you can modify settings only through the object on which the alarm is defined.!"
vSphere 5.5 Documentation Center
Hope this helps.
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