unfortunately , thats not possible with SEPM as of now
you cannot schedule updates
it happens based on heartbeat / push mode or pull mode
there is an idea created for this but not sure when it will be implimented
, need your vote to expediate this implimenation :)
https://www-secure.symantec.com/connect/forums/client-updates-can-be-taken-sepm-121-server-any-specified-scheduled-time