Double-click the taskbar icon to open SecurityCenter
Click Advanced Menu (bottom)
Click Configure (left)
With SecurityCenter highlighted at top left
Click Advanced (right)
Click Update Options (top left)
Change yolur options at the right.
You can't set an actual time for updates but you could have it alert you or simply turn it off.
If you pick the latter you will have to manually check at least once per day or it will start throwing up errors.