Can't update XenServer

I tried using a DVD to upgrade some of my XenServers from 5.6 to 5.6SP2.  I got an error that there were no installations found that could be upgraded.  The solution to this was to apply the update from XenCenter.  It appears some updates have to be applied from XenCenter; some updates have to be applied from an DVD or ISO.  If one doesn't work, then try the other.