Click
'Edit/Delete calendar group' from the Calendar Group Management menu. An Edit Calendar Groups window will display.
You can rename any of the current oganization tags, and you can delete any tags that don't have calendars assigned to them. If a calendar tag does not have a delete check box next to it (indicating it can be deleted) then it can't be deleted because this tag currently has calendars assigned to it.
To be able to delete the tag, un-assign all calendars in this group through the arrange/group calendar interface, and then come back to the Edit Calendar Group window and delete the group.
Note: if you rename a tag, the tag will rename and show the new name the next time the screen is refreshed.